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MG Midget and Sprite Technical - Painting and preparation

A fairly regular topic, I know, but I want to check opinions and experiences.

I am begining on the preparation of the frog's external body panels for painting. The underside, interior and engine bay has all been finished with brush-applied 2-pack but the final finish on the exterior body panels will be cellulose.

I have previously given these panels a light blow over with rattle-can paint just to keep the rust at bay as I progressed slowly with the welding stages. That is easily removed with thinners and then the surface wiped over with a phosphoric acid wipe to get it back to bright steel. But what would now be a good sequence of treatments, and what to use to prepare it for the cellulose colour coats? I do also need to use filler to lightly skim over some irregularities and weld lines.



GuyW

I'm not an expert Guy, but for very small imperfections I think Knifing Stopper (Holts call it Knifing Putty) is easier to use and gives a much finer surface finish than body filler. I expect you knew that though.
Greybeard

Possibly at the later stages, Grey. It will need a bulkier filler to shape up some areas first.

I am really looking for advice on the metal treatments/ preps.
GuyW

Guy, I'm no expert either but my approach is filler (Isopon P38 in my case) on etch primer. Are you using epoxy primer? Some people say filler on bare metal but as I'm using etch primer I don't want it to "burrow" under previously applied filler. I'll need a lot of filler to align door edges at front and rear with rear wings/ A pillars and to generally hide blemishes in rear deck and bonnet. Flat the filler, stopper as needed then two or three coats of high build primer (celly in my case). Then the work begins on flatting eventually using a guide coat. Then several coats of celly colour. I haven't done any of this on outer panels yet but have sprayed high build and colour coats on the interior and engine bay to get the feel of the paint. (primer not flatted). I intend to fit all components including the engine/ gearbox before doing external bodywork. Have you fitted the windscreen yet? For me, this is all several months away yet.



Bill Bretherton

I always used Holts Metallic filler
Although I have had rust getting back into the metal base surface (always when my undertreatment was inadequate) I have never had 'water swell' in the filler which I have had in the past on other less well prepared cars as water got through P38 type fillers to the base metal

I pretreated most of the panels with Trustan, which has only recently allowed a certain amount of rust back in/out

I'm in a 'slow repair-rusted bits' regime now though, I keep finding bits on which the Trustan was just a paint layer because I didnt let the surface rust then give it the treatment but the Metallic itself has still not failed

I am repairing bits I did wrong, not bits where the materials failed
Bill sdgpM

There is more than one way to skin a cat . . . and I suspect there are lots of choices and personal preferences involved. I just don't want to go for the wrong combination of products!

Filler : Like Bill I have much favoured metal based fillers in the past but when I went to buy some from Halfords a few months ago I got the last tin on the shelf. When I asked if they were getting more in he said they had re-branded it as an alloy wheel repair filler. It was a reduced packet size and greatly increased price! I wasn't convinced, but looking on the Holts web site they no longer list the metal body filler in tubs as they used to. :-(

As to when to apply it there is definately conflicting advice. Either directly onto a slightly abraided,(80 grit W&D paper) clean steel so it bonds well. OR over a coat of primer so that rust doesn't creep in and develop under the filler patch. But then, if applied over primer you are you just sticking it to a paint film, which itself might lift off?

Cleaning the metal: I am dealing with surfaces that can be cleaned back to mostly bright clean steel, using thinners to remove the temporary pain and phosphoric acid based metal cleaner. This dissolves off the surface rust pretty well instantly and the metal then dries to a slightly dulled "pewter" colour. As an experiment I have cleand bits of steel this way and then left them outside without any further protection. They do rust a bit after a few months, but the surface is streaky and the dried phosphoric acid solution clearly gives some lasting protection. BUT wht I don't know is if the acid would eventually work its way through to damage overlying paint finishes if I don't neutralise and wash it off.

Despite being clean of apparent rust there is bound to be some in crevices, corners and small pits that won't clean off, even with the acid treatment. I can only think that a rust converter like Bill's Trustan is the way to go, but it doesn't bond well to clean steel so will need to be done as a spot treatment with a small artist's brush so it only goes onto the rusted bits I suppose.

GuyW

You could 'lead load' rather than fill if you wanted.

If using a filler always fill against bare metal and use the best quality filler you can buy.

Brush prime over the filled area and once dry skim with cellulose putty. After painting you can use further very thin coats of cellulose putty (also known as 'stopper').

The brush primer and cellulose putty are both to prevent pin holes appearing in the filled area and to remove and final imperfections that were not visible until paint had been applied.

The cellulose putty is flatted down with something like 220 wet (and dry) paper with a bit of soap in the water to prevent the paper clogging.

From memory...
Daniel

Bill, I have used etch primer in the past, but it was very thin - more a sort of wash than a coat of paint. Reading the adverts for etch primer now they seem to be intended for priming galvanised or alloy surfaces and for plastic parts, raher han for providing a bond to steel. The standard (non-etch) primers are advertised as having good bond characteristics for steel surfaces.

I am not sure about the polyurethane primer. As you know I used it on the underside and engine bay and it does provide a good hard base. But I found it a bit "gloupy" to use - I was brush painting it - and the final surface wasn't smooth, and if fact in places was distinctly gritty. The big advantage is that it is fully waterproof so you can leave the car in primer over damp winter months without condensation penetrating to the steel underneath. Of course once your primer is top coated with cellulose it is waterproofed anyway so this advantage no longer applies.
GuyW

You can't leave phosphoric acid on the steel. Where it has contacted rust it will have formed iron phosphate and that should be stable, but any trace of the raw acid will come through the paint. Wash with water and dry thoroughly before applying anything.

Ordinary fillers like Isopon and Holts are not waterproof so if any dampness can get at them from above or beneath they will absorb the water and swell. While working on a Lotus Elan shell I was introduced to "Watertite" epoxy filler, made by the International Yacht Paint people. Designed for boats, this stuff is brilliant. It is creamy smooth and once set is totally waterproof. It's a bit pricey but then most good things are.

Whether the filler should go on the bare metal or on a primer is a tricky one. I suppose it will be OK on the primer as long as the primer is very firmly bonded to the metal beneath, which an etch primer should be. The trouble is the filler should ideally have a key of some sort to enable it to grip firmly. Bare metal can be lightly scored to give that key. Paint is much smoother.
Mike Howlett

My understanding now is to rub down filler dry only, no water. It absorbs water like a sponge and you don't want that surely. Same with high build celly primer.
Bill Bretherton

Thanks Mike, that answers my question about the phosphoric acid. I do have some neutralising wash to use.

And thanks for the tip on "Watertight" That sounds like a good replavcement for the NLA Holts metal filler. I will have a look for that. Using Isopon and U-pol fillers is fine if you are working in a heated professional body shop, and probably completing the whole repair within a day or two at the most, but no good in our damp northern climate!


Just seen your post Daniel. Yes, I have done some lead loading already - more for the fun of it although that is what was used on the original Sprite shells at the base of the screen pillars. It was surprisingly easy, considering people consider it is something of a "black art"! Once I have got the initial metal prep and primer on then I will be using high build primer, knifing stopper and colour coats


Bill, I find that used dry, the paper clogs almost instantly. It does seem better to use waterproof filler in the first place.
GuyW

Yes, the paper does clog so I understand why people use water. My problem is the filler is surrounded by etch primer which I don't want to get wet either. My shell came etch primed so I continued to use it on other external panels. Maybe sprayed epoxy primer might have been better. Waterproof filler sounds good as long as it rubs down as smoothly as P38.
Bill Bretherton

"International Watertite" filler £26 for 250ml. Hmm, yes that is a bit pricey!
Though a litre is "only" £60 so a bit better.
GuyW

For some reason DA sanders don't seem to clog up too quickly, and they are used dry. Maybe the time has come to treat myself to an electric DA sander. ;-)
GuyW

I don't have a DA sander either Guy. I suppose I'll have to get one too.....
Bill Bretherton

Tough, isn't it Bill !
GuyW

What are the two actions?
Nick and Cherry Scoop

I think it is rotary superimposed on another rotary i.e. it turns on an axis which is, itself, rotating about a point. I'll happily be corrected on this.
Bill Bretherton

I bought my Frogeye nearly 40 years ago and the only thing on it that was done well was lead loading of the bonnet seams. They are still perfect today. I've never done it myself but can vouch for its durability.

I find that cellulose stopper is prone to shrinking, so prefer acrylic stopper.

Les
L B Rose

I'm nearing the end of repairing and re-painting one of my doors and have learnt a few lessons along the way.

After being warned off air sanders due to lack of compressor capacity, I bought an electric Makita DA sander but made a typical 'school-boy error' - if buying a DA sander make sure which size you're getting, mine is 125mm. There are plenty of 125mm discs available - but they're all 'sand-paper' for use on wood etc. Nearly all the very fine grit discs that you'll want for filler and paintwork are 150mm. There are 125mm ones out there but you'll have to hunt to find them. I've got away with using 150mm discs but the dust extraction holes don't line up.

I used P38 on bare metal for the initial fill on a joddled joint then P40 as a second coat and on a couple of other small dents and a rust pit topped off with a fine stopper. The etch primer sank a bit over the filler so it took several coats of primer filler to make them disappear completely.

I was determined to keep the filler to the minimum but this was a mistake as, now that the top coat is on, you can still tell where the joddled joint is for about half its length but not in the other half where the filler was a bit thicker. Not enough time left before I need the car to start again so I'll just have to live with it for a while. I had planned to do both doors but I've taken so long over the first, the second, which was not quite so bad, will just have to wait.

I used Lechler paint products from my local supplier with Mac 5 Macrofan 1K for the topcoat. Despite what the supplier said, this definitely needed filtering as there were several particles apparent in the first top coat which needed rubbing down and a lot of paint sediment left in the filter on subsequent coats - this was despite plenty of stirring manually and with a home made stirrer on my drill.

Pinching a heat lamp from the dog's kennel definitely speeded up the paint drying process allowing me to put several coats on in an afternoon.

I was hoping to get the door back on the car today but, having taken yesterday off to go to Race Retro (Hi Dave, very glad I found you and thanks for the diff!) SWMBO thought it would be a good idea to chop a tree down, so now I'm pooped and the door will have to wait a bit longer!

Colin
C Mee

I'm in the camp of applying filled to clean metal only and using metal based fillers... sorry to hear they are discontinued ... if that's the case as a pure guess maybe fiber glass resin withowithout the glass

What I hate about the pink bondo is it attacks water like sponge in the atlantic, so I always like to use it as a last coat filled cuz the metal based fillers are so difficult to sand and develop smooth

Id wash the acid off, less and fewer beets in the stew the better chance the taste of the stew will NOT taste like dirt


I'M A huge believer in putting any extra cash into the best primer you can afford to purchase for the last coat before painting... I want to accomplish 2 things with primer... locking in everything that's under the primer so the entire project has a consistent same finish for the paint to stick to

#2 paint adhesion and performance... The tighter that paint sticks to the primer the better if you do that the paint will perform better it's easier towash, holds was better and is much more resiliantto age and abuse

When applying the 1st coat of filler, I don't like a slick super smooth surface.. I like it a little scratchy so the Hondo can really grip to the bare metal

For cleaning areas that are hard to get into tight spots and cravouses I like spray can brake cleaner it attacks any kind of gunk, it's under pressure so it lifts any dirt out, and the acetone in the can flashes off fast leaving a clean spotless non residue finish

Luckly... I don't do metal painting like this very often
1 Paper

If you work in an unheated garage any prep you do now is likely to absorb moisture at a molecular level and probably come back to bite you. Etch, epoxy and any fillers and primers all absorb moisture in the air to a greater or lesser extent, so there is no safe barrier you can apply and leave to topcoat later in the year. Pro paint shops are temperature and humidity controlled for this very reason - and even they manage get it wrong when the car isn't adequately force dried, so the prep conditions need to be almost ideal to be right. If you check the temperature and humidity today it's around 2 degrees with 65% humidity. This is so far away from 60 degrees and 15% humidity you need to have the car at, and usually only seen for a few days at the height of summer. Outside of these figures you can almost guarantee the paint will erupt - sooner or later - with micro blistering or pickling as that trapped moisture comes to the surface. Traditional wet sanding although cheaper than dry sanding, will stay in the substrate unless you force dry. That doesn't mean waving a hairdryer over the panel, as you need to get everything up to temperature, even down to the lines and vapour traps.

The best chance I think you have is to prep the steel, panel by panel and seal it in gloss rattle cans as a temporary measure. Plan to finish the BIW by July and when your temperature/ thermometer on the garage wall reads over 50 and less than 25, commence the surface prep starting with UPol and acid etch and straightline sand the whole car for a good month, correct all profiles, gapping and rads to all edges. Fill pinholes with fine polyester filler so the substrate is all the same animal. Then it's primer, a contrasting guidecoat, and more blocking down. Check your door swing clearances at the A Post and attach the check strap. Buy How to Paint Classic Cars and look at paint tutorials on YouTube, as watching someone painting a car is worth more than any description. Check the weather forecasts frequently and if D Day is a Wednesday in August plan to make the garage a dust free zone ( hang clear polyester sheeting) and damp the floor. You'll need really good lighting (rows of strip lights are best) plus extraction, or paint outside if you need more space.

I tend to do the closures first, underneaths and returns, then tweak the fan and adjust the pressure and do the sides, followed by the rear shroud and the bonnet last. In BRG I get full coverage in 3 full coats, but lighter colours will need more.The trick is to control the gun at a speed and volume that atomises the paint to maintain a nice wet edge. Buy more paint and thinners than you need, always from the same supplier, practice on other stuff first, and be super meticulous stripping and cleaning the gun at the end of each session.
f pollock

what he said above ! great and informative answer.

All i would add is to be realistic.

I have only painted two cars in cellulose, and I am self taught, so i know it can be done at home in a garage that in my case and perhaps like others on here has multiple uses.
That said, clearly the better the prep the better the finish, but one thing on your side is that with Cellulose you can flat and polish away once done, and provided you have enough paint on the car, you will be able to get a very passable result.

Ensure you have plenty on the edges of panels, doors etc as these are the areas where it is super easy to flat or polish through.



P Bentley

F Pollock, do you actually mean 15% humidity??! We never get that in the Uk. My understanding, after extensive reading, is that 50% or lower is ok for cellulose and 21C ideally. But it's never ideal, is it, especially up north (where I'm from). So we surely have to accept a compromise. Anything under 55% humidity at between 18 and 23C would be ok for me and, if you're lucky, you'll get that over the few days window you need to do the job.
Bill Bretherton

Over the years I have successfully sprayed 4 full cars in celly and one in 2K (which I didn't like using and it wasn't as successfull). Plus a lot of individual panels and part re-sprays. I have always managed to get what to me was a decent and acceptable finish and probably under rather less than the ideal conditions that Fergus rightly describes! The worst was my yellow midget, sprayed outside on a summer's day. The yellow colour attracts small insects from miles around so it needed a lot of fine sanding and buffing to get rid of the remnants!

What I was really asking about was the preparation stages as there seems to have been greater changes there in products and treatments. I think that there is more to be won and lost in choice from the currently available early prep stage products than in the top colour coats.
GuyW

Fergus, please don't mis-interpret my comment re. humidity. I, perhaps, didn't make it clear that I realise you were describing ideal conditions which, unfortunately, we never seem to reach in the Uk.
Bill Bretherton

I've sprayed a number of cars in celly including my frogeye in winter and early spring conditions and found that a 3kW fan heater run for a few hours before the spraying warmed the air in my mother's double garage nicely and I've never found any issues with the subsequent paint results. I've always been bold and sprayed near the edge and rarely gotten runs or curtains and the finish from the gun has been excellent and think that is better than holding back and having to flat back orange peel because of being cautious. It can be worth enquiring about badly blended paint which may be available on the cheap for practice purposes as the mixing schemes may cock up occasionally, with celly not being so common these days it may not be so common but back when it was more common I sprayed a girlfriends push bike in what became known as Pageant Blue cock-up.
David Billington

15% humidity is tough just about anywhere around the equated of the planet

I once had to stain some specialty wood trim with a specialty stain i had never seen before for a movie theater and it called for 15% humidity at a fairly high temp, at the time I was joking I needed to load up and go to death valley Arizona, to stain all this wood to match those type of conditions and just laughed as did everyone else

5 weeks later... it sort of dried enough that we finally got to install the specialty trim in the movie theater.... haba augh

Prop
1 Paper

Bill yes I'm describing ideal conditions, but we do see high 20's and 30's on a few days - usually at 2.35 in the afternoon. Over 50% and you really need to use anti bloom thinners - which is OK but the shine out of the gun isn't as good. David warming the garage for a few hours is effectively creating those conditions with his fan heater, No wuzzers, lots of space and Mum ready with a nice cup of tea. Probably as good as it gets.
f pollock

I'm presuming that the ideal initial preparation is to media blast the body (expensive but effective) then immediately etch or epoxy prime. Then fill as needed, rubbing down primer before filling. Then re-prime where needed, after which move on to high build primer etc.

I had the rear deck, a rear wing and the doors media blasted and it does give a superb prepared surface. I had the bonnet/ front end acid paint stripped before beginning work on it. I temporarily etch primed it which I'll re-do when welding is finished. Might have it media blasted but it's about £200.
Bill Bretherton

An hour ago we had 25% here on the border. Now it's 34%. I wonder what it's likely to be in my garage, which is warming up now while I gather my courage.
Nick and Cherry Scoop

Humidity here in Cumbria is 62%, which seems quite low, but then it is a bit chilly- just on freezing. As I understand it, warmer air carries more moisture so the humidity level is likely to rise if simply warmed up. What is needed is a bit of air stream from the Sahara in the summer (minus the dust of course).

Virtually no snow here is S Cumbria, more green in the fields than snow. I do think the Met office (and BBC) are in danger of "crying Wolf" over weather warnings these days. Reports of heavy snow in Newcastle, accompanied by photos of about 2" of snow are just ludicrous!
GuyW

Guy, I totally agree with you about weather warnings. There seems to be as many of them as there are high vis vests these days. The problem with that is that they become so regular that nobody takes any notice any more.

It really gets me when the police say dont go out unless you really have to. The best way to keep the roads clear is to use them as much as possible unless there are huge drifts of course.

Trev


T Mason

Guy,

My understanding is warmer air can hold more moisture but the humidity quoted is relative humidity, https://en.wikipedia.org/wiki/Relative_humidity , and is expressed as a percentage of the moisture the air can hold so heating the air will drop the humidity unless you provide a source of further moisture.
David Billington

Didn't realise that David. But it makes sense. So earlier, when Fergus and others talk of humidity needing to be less than 15%, they are meaning Relative Humidity? Still likely to be unobtainable in Cumbria at any time of the year!
GuyW

The type of heating will make a difference too. Electric will be dryer than say calor gas.

Trev
T Mason

Trev,

I was thinking that as well, burning fuel would be pointless unless the combustion products can be vented outside as otherwise you're just adding CO2 and water vapour into the space. Maybe if you had a heater with a heat exchanger like they often have in factories but a normal gas space heater would be a waste of time IMO.
David Billington

It has only just occurred to me but as many people with garages attached to the house have modern boilers installed in them why dont they have radiators put in them. I cant recall ever seeing a garage with any in.

Trev
T Mason

Trev,

It's what a mate has planned for the new garage/workshop he's adding to the side of his house so it can be warm al year round. The old lean to it's replacing was just a drafty covered space mainly used for storage, he already has another double garage in front of the house as the main workshop space.
David Billington

I can confirm what David says about calor gas heaters. I bought a fan assisted space heater for my garage many years ago. Marvellous thing, I thought, 10 kw of output. When I tried it it heated the garage really well, but everything became damp and I got a headache from the exhaust. It hasn't been used for years.
Mike Howlett

Any device burning any form of hydrocarbon with release large quantities of water vapour into the air as part of the combustion process. That is, unless it is designed so that ALL the combustion gases go direct outside as with a balanced flue boiler or a closed wood burner where all of the smoke up the chimney.
GuyW

The good news ... winter has almost ran its course and warmer temps are really nice

Bad news the rains of March start tomarrow and will last well into may... Augh


If you don't like missouri weather.... wait 5 minutes

Prop
1 Paper

Prop,

What did punxsutawney phil have to say about the weather prognosis this year?
David Billington

Trev,
if you have a boiler and or rads in garage (and dehumidifier) that have water in them you'd need to have them fitted with a frost stat so they don't freeze up and burst so they would be coming on when the rest of the property is well above the temperate in the garage. Depending on the number of rads and pipe runs it would add a lot of colder water to the system unless you made the garage rads a different zone with perhaps motorised valves.

As Guy put balanced flued domestic "wall heater(s)" assisted or unassisted air convection heaters were a good idea for a garage or commercial models for those garages that hold more than a couple of RRs(all subject to available flue space).
Nigel Atkins

This thread was discussed between 24/02/2018 and 27/02/2018

MG Midget and Sprite Technical index

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