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MG Midget and Sprite Technical - Filler

Filler - I have used the stuff for well over half a century but keep coming back to the same doubt in my mind.
Is it better to apply direct to bright steel, then sand, prime etc.
OR, should I etch prime the steel first, then add the filler? This would seam to give a better protection to the steel, but OTOH the filler is then just bonded to a paint film, and reliant on that for adhesion. I have read of both methods, but what's the view here?
Guyw

I end up doing both generally.

I will start off with bright steel, but if it is a larger or more complex curved area, I will often put a coat of primer on midway through. This makes it easier to assess progress, low and high spots. More filler inevitably ends up over low spots with primer in.

But that's just what I find works for me. I have never had any expert tuition on the matter.

Cheers,
Malc.
Malcolm Le Chevalier

My logic says filler is porous so any later break or scratch in the paint will eventually allow rust to start on the metal which then spreads and pushes the filler off. Etch primer first would seem to counter this.
David Smith

David, that's my gut feeling too. But then the paint adhesion issue began to get at me and I had doubts. I know I have done it both ways in the past.

Just interested to hear how others see it!
Guyw

We were thought in auto body class to apply the bondo on to bright shinny metal with NO rust primer or paint

But that was 35 years ago ...i think the reasoning is the primer /paint provides some kind of ledge that condenstation during radical tempeture changes can attach to that wont occure if its just direct bondo to metal

bla bla bla

prop
Prop and the Blackhole Midget

That's an interesting point you make, Prop. I have often come across interstitial condensation - to give it its posh name - where it can be a real problem in building construction. Never thought about it in car bodywork though.

Didn't realise that you had done an autobody class. Sounds like an interesting opportunity, even if you were only 6 at the time! LOL
Guyw

Guy
He may have been 7....but with the toxins Prop cant quite remember.

What about contacting the manufacturer of the filler...or looking up their recommendations on the web and reporting back to us? Just an idea...

Hey look at this!

http://bondo.com/tips#MEqbwIkeqQA12Tyq.97


Here in the states they call it bondo!

Once applied on your car...it becomes a bondo bucket!
S

In the mid 70s I worked for a guy who repaired/broke BMWs (before they became sooo common) and he always applied filler to bare metal.
He would also join two halves of a wing (to make one good one) by gas welding - quite a technique to watch as he never distorted the thin steel.
J Tickle

I've only seen filler applied to bare metal. But from what I've read, etch primer bonds to metal chemically and physically, so why should it separate?

I did not know till today that it only starts to etch when it's dry. Interesting, and surprising, and a dire warning to impatient painters.
Nick and Cherry Scoop

That's very odd Nick. It,s based on phosphoric acid isn't it? Why would it only etch when dried.

Surely it must be the other way round? Etch when wet and then stabilises as it dries?
Guyw

Come on... lets not exaggerste... i was 9...haha

there was a time that i did want to be a body man... i did 2 years in high school tech class to learn the craft and took me about 6 months after grad to say screw this bad idea

theres the correct way to do body work like we learned in screwool and then there is real world way... and EVER Freaken moment in between involves lots and lots and lots and LOTs and horrendous amounts of .... sanding and not just any kind of sanding... wet sanding augh

nothing like a cold shop at 40 degrees F in the dead of winter and your sitting on top of your head WET sanding the under side of a rocker panal


primer in general is strange like that at least in home improvement.

it gets harder over time... some primers if you let them sit for 10 days before getting the over spray off the wood work your better off to just replace that section of woodwork then to mess with trying to remove the primer over spray

thats the 1st lesson you learn as a house painter ... always carry a clean rag with a wet courner off the edge of your belt... rogue paint is alot easier to clean now then it will be 5 minutes from now.

prop
Prop and the Blackhole Midget

A filler / bondo i really like using on the midget is whats known as (al-metal)

Its more expensive then gold oz per oz ... but it flows and trowels very nice and curers faster and dosnt have near the pitting of the pink stuff...and it resist moisture alot easier but its alot harder to sand also

So used it as the base filler then used the pink stuff over it for the 2nd and 3rd coats

its a resin like bondo but its some kind of fine metal flake added to it ...maybe aluminume dust for lack of a better term

prop
Prop and the Blackhole Midget

I agree it's odd, Guy. I'm almost entirely ignorant when it comes to painting, so I just googled etch primer and read the Phoenix Paints entry. Right down to the bottom of the page, where it says, 'whilst all information is believed to be correct . . . . . . . . . . '
Nick and Cherry Scoop

Guy
I put it on bare steel e.g. my Frogeye doors had been paint stripped and then etch primed to hold rust back. When I repaired the holes I removed the etch around those areas, welded repair patches where needed then filled onto the bare metal repair. I then primed the repairs with zinc primer as, from reading various sources, it seems there's a chance that re-finishing with etch primer might lift the filler as it tries to burrow under the metal around the filled sections. Just a thought.
Bill
W Bretherton

interesting question OP.
I apply the filler straight to the bare metal, then prime it, but that's not because I think it is any better a method than vice versa, its just the way I have done it.
My nephew works at a Porsche restorer so I will ask him what method he works too, but on the youtube videos I have watched I seem to recall its filler straight on to metal.
I will post again once I have heard back.
P Bentley

Helpfull discussion. I have been unsure of this for years, and have done it both ways. I can see that a paint film under filler might lift off, but thought that perhaps an etch coat that chemically bonds might not do this. Plus the etching that I have used is extremely thin - its like painting with water, although they ae not all like this. Some etch primers have more "body" to them which might be a problem.

The consensus here is coming down on the side of direct onto steel so maybe that is best.

Just to complicate the issue though, I would expect an etching coat or mordant solution would be needed first if it was being applied to a galvanised surface.
Guy Weller

Well my nephew has replied and apparently they apply a thin coat of red oxide primer then key the surface, then add the filler.
I have used Bonda Primer for the mini I am doing which is reasonably thin and easy to apply, so hopefully that helps us all out.
P Bentley

I may be wrong but I seem to remember reading somewhere ages ago that a reaction between the metal and the filler causes heat which helps the filler to cure.

I haven't looked but what do the instructions on the tin say?

Trev
T Mason

Link from U-Pol for Isopon P38


www.u-pol.co.uk/documents/datasheets/tds/P38-TDS-EN.pdf

Well it didn't link direct - cut and paste.

Both seem to be O.K.

R.
richard boobier

Trev

you are correct...curing is a chemical reaction and not an convection aka air dry method

depending on the ratio for part A hardener to resin part B the reaction can get very hot almost untouchable.

in the winter time we made hand warmer out of the stuff as long as the shop owner or teacher didnt catch us....haha

prop
Prop and the Blackhole Midget

Prop, I know there is a chemical reaction between the resin and the hardener but I was talking about a reaction between the mixed filler and metal. Speaking of the ratios of the mix it is also not a good idea to add too much hardener as this can result in cracking.

Trev
T Mason

Thats my understanding on the hardner

I was taught a baseball size resin a golf ball size of the white part b and a tooth brush amount of the hardner ...

also dont mix in a fast curcular motion but more of a folding over into its self motion as that will give it a better mix and less air pockets aka less pin holes

also mix on a sheet of tempered glass and not card board ... cardboard absorbs the chems and weakens the mixture..were as a sheet of glass wont abskrb and its easy to crean off ...same with mixing tools they should all be plastic

im nof aware of a chem reaction from metal to bondo...but thats not to say its not true.

and as alway... less is more when applying

i always liked to apply less then i thought id need and add in thinner coats to the level that was needed rather then a huge 1st coat that took all day to sand down into shape... anything to avoid sanding

also apply very focased and centrialized the 1st coat then broaden out the aera with each additional coat while making each additional coats thinner then the one before

OR do it like steven and I were probably taught by our red neck uncles... mix the whole can at one time sling that mess in the big ol dent smooth it out with a beer can... let it dry smooth that over with bouse joint compojnd put kilz primer over the top ... use about 5 tubes of spot puddy and get some krylon rattle can paint and spray that thing down before 3pm so we can eat bbq at 5pm ... hee haw

if you drink enough pabst blue ribbion ... you will never notice all the sand scratches and imperfections

prop... the bondo hillbilly expert
Prop and the Blackhole Midget

I just checked on a can of actual Bondo brand bondo, and the directions say to put it on bare metal. I once tried to put it on top of the spray on rust converter that turns the rust black. When I went to use the cheese grater file on the bondo, it all peeled off in sheets right back down to the converter. It is possible that I didn't wait long enough between the converter and the bondo.

Years ago I was repairing the rear quarter of a 70s era Buick station wagon that had the vinyl fake wood trim on the sides. I found that it had been repaired before because there were three layers of the wood vinyl with bondo between the layers. So, it wasn't done down to bare metal.

Charley
C R Huff

We add chicken wire to the big holes..... to add strength for our repair here in the salty covered roads of the northeast! LOL!
S

When l got my 71 car the rear quarters behind the wheel arches were filled with concrete !

I better add, though l am not sure you will all believe me, l am only using a very thin skim of filler to smooth over over a couple of welded seams!
Guy Weller

It's maybe worth pointing out that USA Bondo and filler over here (of the isopon variety ) are two different things. Bondo is a smoother filler and works for longer - it can be used to cover an entire panel in a very thin layer (watch episodes of American hotrod to see what I mean). Our fillers tend to be a bit 'heavier' in texture and won't cover a full panel - e.g. a door - before it's started to go off and becomes unworkable. I tried to get real Bondo when I restored my car as I was going for the boyd coddington 'smooth as glass' look but was unable to. Took a lot of wet sanding of p38 and leaving all the sanded panels in a heated room to get all the moisture out.
graeme jackson

Out of interest does anyone on here use cellulose putty to finish off? Ive not had to do any bodywork for a while but its something I always used to use to get rid of those small imperfections.

Trev
T Mason

I use cellulose or 2 pack stopper to fill small imperfections as I find it sand far easier than filler.
David Billington

Oh those arw great bondo stories

we have whats called spot putty...it comes in a tube and you just squese out a dab onto the imperfection and force it down with a putty knief... tbh i think its just childs play doh just watered down a bit more to make it flow better

prop
Prop and the Blackhole Midget

Guy...now that is funny i bet i know what happened

years ago i had a 1 ton cargo van for my paint truck... i did the body work but had a huge rusted out hole and i didnt want to spend the time to fix it correctly

so used 20 minute home joint compund ... its not an air dry compond its a chemical used mainly in houses on old plaster ... but its super strong and wont crack and as you say ... looks exactly like concrete and feels almost the same .... so stuffed that in the hole and put bondo over the top and zip roll painted the truck the next day... with exterior house paint of course... held the entire 5 years i owned it

but i forgot about that... i bet a wooden nickel thats what that was...fast simple and cheap

prop
Prop and the Blackhole Midget

I have found the best and smoothest filler is not designed for car use at all, but for boats. It is made by the International Yacht paint people and is called Watertite. It is a 2-part epoxy filler - you simply mix equals amounts of the two parts. It is super smooooth and is waterproof when set. It sands easily to a very smooth, baby blue finish. I love it. Have a look here
http://www.yachtpaint.com/irl/diy/products/fillers/watertite.aspx
Mike Howlett

Re. International's 'Watertite' filler. Fascinating, thanks for that suggestion. Worth investigating. Certainly, as a brand, International did make some very good paint (their Yacht Varnish was superb). Presumably the company has been swallowed up by a conglomerate more recently.

At over five times the price of bog-standard polyester filler, I question whether the cost of 'Watertite' is justified? I think the biggest problem with any of the fillers is moisture absorbtion that leads to the filler swelling. If one can seal the metal/filler/paint interfaces sufficiently well, moisture should be able to be kept at bay. In theory.

And using any of these fillers is only (or should be 'only') for cosmetic purposes.

Circa £25 for a 250ml tin of Watertite. It bloody well better be watertight. LOL

Andy Hock

The so called bog standard filler (bog) goes on ground bare metal-sanded, then primed then spot putty or spray putty if needed then sanded, primed and painted
Remember all of these will absorb a certain amount of moisture and should be painted/finished as early as possible-----don't go driving around for ages in primer as this is when the corrosion under the paintwork starts
willy
William Revit

Hmmm I didn't know that...I thought the primer sealed up the metal...live and learn
S

Another thing about filler. I don't think you should wet sand it because it absorbs water like a sponge. You certainly shouldn't drive around with unpainted filler on the car like many seem to. Same goes for cellulose primer. Cover with top coat even if it's only temporary.
Bill
W Bretherton

I love that idea of water tight filler... thats a great idea
Prop and the Blackhole Midget

This thread was discussed between 23/08/2016 and 29/08/2016

MG Midget and Sprite Technical index

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