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MG Midget and Sprite Technical - Dealing with pits

I'm intending to smarten up my sills later in the year.

How do folk go about dealing with surface rust pits? Apart from this pitting, the sills are solid, having been replaced some time before the Sprite came into my possession.

I'll probably strip the sills back to bare metal before spraying rather than just spraying more layers on top, but I would prefer not to have a textured bottom edge.

After cleaning up as much as possible, treating the rust and etch priming, I'm trying to decide between two options.

Option 1: Build the surface back by spraying with high build primer/filler with a number of layers, sanding back each time until I've got an acceptably smooth surface and then top-coating.

Option 2: Apply a thin layer of fine surface filler before sanding back to smooth, probably in more than one layer.

Or can you suggest further options?

I'm not looking for a concours finish as I doubt whether I could achieve anything close, but I would like the sills to look a bit smarter than they do at present.



C Mee

Colin
I would use filler first. High build isn't really intended to fill in rust pits - it's more about enabling a truly flat or even surface to be obtained after sanding (using a guide coat ideally) so that the colour coat looks good. High build thickness is less than you'll get with filler. You might be able to get smaller pits smooth with stopper (after priming). If using celly, slight blemishes are harder to cover up than with 2 pack.
Bill Bretherton

I agree with Bill, resin filler not anything solvent based. But vital to get all the rust out of the pits - mechanically or chemically (probably both). After the filler, I use high build non-toxic 2K primer. Great stuff, dries very fast and they say you can top coat after 2-3 hours. I chickened out and waited until next day. I got my paint from Ken Burdekin. It makes a big difference to use good quality primers.
Les Rose

Whatever else you do make sure that any rust is both mechanically and chemically removed to ensure all traces of rust are both removed and converted and thereby removing any moisture that would lead to further corrosion occuring under the new paint.
S G KEIL

Not especially for paint preparation, but about 25 years ago the rear of my diff was so deeply pitted with rust that it was beginning to weep oil through the pores. I degreased it and layered a skin of 2K epoxy resin over it, sanded and repainted. It cured the leaking and still survives perfectly intact.
GuyW

Thanks for your comments.

I always try to get rid of any surface rust mechanically and then treat all bare metal with Kurust before etch priming.

I've been very frustrated with the fine surface filler that I've been using, which must be solvent based as doesn't have a separate hardener. What 2K fine fillers do you recommend?

Les, can you give me the name of the 2K primer that you use?
C Mee

Guy, I had a midget 1275 that weeped oil through the back of the diff casting - I had never heard of such a thing happening and thought mine was unique. I did the same as you with a 2-part filler.

Colin, there are many 2-part epoxy fillers out there and I suspect any of them will do. The best one I have found in my limited experience is International Paints Watertite. This is designed for boat hulls so is completely resistant to absorbing water, and it sands down beautifully smoothly. Like you, I also have had poor results with one part "cellulose" fillers. They always seem to sink over time, even when the layer is very thin.
Mike Howlett

Agree with Mike re: International Watertite. That's what I use on the boat. Mostly on GRP where people have been a bit casual manhandling the 60lb anchor but it works great on metal too.
Two things....
You need to be meticulous about degreasing. I use neat alcohol. And Watertite has a very short pot life until it's unusable so if you mix a lot at once it gets a bit wasteful.
Cheers.
Greybeard

Ive had a lot of success with Phosphoric Acid (available from eBay) on cleaned up rusty metal. I use about 50% solution. Clean up metal, apply acid, leave till dry. If any rust colour remains reapply acid and leave to dry. When satisfied clean with Panel Wipe and apply your favourite covering. I prefer Etch primer or the Epoxy primer that has an activator.
Rob
MG Moneypit

Another product I've used on scaly de-rusted metal is Vactan which supposedly neutralises any remaining rust specks and turns it bluish.
Bill Bretherton

Not so much to do with filling rust pits, but for when you want a fine skim of 2K filler: I found a very big difference between the products sold by Halfords, Isopon P38, I think the call it, and the U-pol finishing filler bought at the trade counter of a proper paint shop. The professional quality product went on much more evenly didn't drag with the spatula and when it came to sanding it was so much smoother and easier to get an invisible feather edge.
GuyW

I agree with Guy about filler. I switched to Upol Fantastic and it really is, err, fantastic. They also do a finishing one as Guy says. Trouble is you have to buy a camparatively large and expensive tin with limited shelf life (ok if you've plenty of filling to do of course).
Bill Bretherton

Thanks for your comments, they've been really helpful.

I've also been going back and forth between whether to sand back to bare metal or use chemical stripper.

I've got Starchem Synstrip, which does a good job but, if the newly exposed bare metal isn't treated quickly, surface rust begins to appear almost immediately.

So my next question is which would you recommend, Strip of sand?
C Mee

Bilt Hamber Deox-Gel is the most amazing rust remover I've found. It's not acidic, not toxic. You dollop the gel on and leave it for 1 - 6 hours (depending on temperature), using cling film to prevent it drying out. Then before cleaning it off you wire brush the area, then you wash with water. Hey presto, the rust is gone and you have shiny metal. Repeat if the pits are deep. I've no idea how it works but it does what it says on the bottle.
Mike Howlett

Colin, I used 2 primers:

Upoxy Superetch

U-Pol 20:20

See photos. I got them from Ken Burdekin:

https://www.carrestorationpaints.co.uk/





Les Rose

Sill pits update.

After spending the last month that got filled with delays working on and off on the sills, I finally got there yesterday!

I stripped the sills back to bare metal using a mixture of Synstrip and sanding. I've had the Synstrip for several years and its more volatile components have evaporated off so it was not as effective as it used to be.

The pitted areas got a thorough wire brushing and then treated with Kurust.

Then came the filler. I used Lechler Multilight 2 pack which was on a completely different level from the P38 and 1K filler I'd used previously. It was so smooth it was like spreading cream. I had numerous filling and sanding sessions in attempting to get the best possible finish. Not helped by not using enough hardener with one batch. It did a really good job in the pitted areas. Inevitably more filler ended up in the bin or as dust as each time I over estimated the amount needed. Along the way I found that my outer sills are far from flat, in fact they undulate along their length. I filled a couple of the more obvious depressions beneath the A posts but decided to leave the rest as they are as I'd probably still be filling and flatting in August if I tried to deal with them.

I went with the Lechler products for all the fillers, primers and top coat as that's what my local supplier carries and I didn't want to risk unpleasant reactions if I mixed manufacturers. My local supplier has been very helpful and free with advice - plus the odd freebie - so it made sense to stick with him - and Shropshire isn't over-endowed with paint suppliers anyway.

Spraying the etch primer (Lechler Monoprimer) and primer filler (Lechler Monfiller) went pretty well with only minor blemishes to deal with. I thought the top coat (Lechler Macrofan MAC5) had gone on pretty well so was more than a bit disappointed to find a few runs. I put that down to slightly over-thinning and not adjusting down the gun's flow rate quite enough. I'll wait a few months for the paint to fully harden and then try tackling the runs - I've found some interesting Youtubes showing how runs can be shaved with a razor blade.

Thanks for all the helpful advice. I think I'm ready to tackle the boot lid now!

NB. Camera angle carefully chosen to avoid blemishes - that's not a run, it's the reflection of the cable for a small LED floodlight that has been incredibly useful for this job.

Colin



C Mee

Looking good Colin. If it were me I wouldn't leave it for months. Runs are easier to deal with while the paint is comparatively soft. Couple of weeks will do imv. I'd also colour sand and compound it after maybe three weeks.
Bill Bretherton

This thread was discussed between 23/02/2022 and 03/06/2022

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