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MG Midget and Sprite Technical - Filler or paint?

Pretty basic question, this, and I am sure I have done it both ways. But what is the current consensus - if there is one?

Blending in some new welded steel I need to add a skim of filler. Its only a thin skim to hide a joint but does the filler go onto bare steel, or do I etch prime the steel first? I suppose the same question would arise on the back, where I will need to seal out the damp with seam sealer.
GuyW

I may be wrong, but as much as I read filler should be applied to bare metal, my own logic dictates that I coat it in primer first, to prevent future rust. I've always done that, and none of my filler has fallen of as a result.
anamnesis

As I said, I have done it both ways but probably couldn't now remember which way on any particular job! Filler onto paint would require a good paint bond which is why I thought of etch primer, but I could convince myself either way was best!
GuyW

Etch primer is porous, as is filler.

Epoxy primer is not porous and makers recommend application to bare metal.

I use bonda primer first on bare metal because it's not porous.

anamnesis

Have a read of this Guy.

https://www.frost.co.uk/when-to-use-epoxy-primer-on-a-project/
anamnesis

https://www.ebay.co.uk/itm/184921812467?hash=item2b0e32edf3:g:FtAAAOSwLiVg4aXt

PRODUCT FEATURES
Excellent adhesion, also on "difficult metal surfaces" such as steel
galvanized, stainless steel, aluminum, etc. Quick-drying, easy to process. It has good
filling properties, good flexibility, hardness and anti-corrosion properties. May be
covered with most of the available paints and varnishes.
anamnesis

Good artical, Anam. It covers several points of concern!
I used a 2k epoxy primer for the whole of the underside, interior and engine bay on the Frog, principally because of its waterproof qualities on a slow project that needed preserving through damp winter months. I know it bonds really well but was less sure about how filler would bond to it's hard shiney surfaces. But this, from that link, answers that point!
"Alternatively, you can apply filler or primer surfacer over the epoxy to fill any minor imperfections and block flat before you paint."
GuyW

Currently applying filler to my project and finding that with the amount of sanding I’m having to do to flatten the area, I am sanding the paint down to bare metal around the edges.

Seems a waste of paint to me, but had to paint to stop it rusting whilst I finished the body work

James Paul

My understanding is that, if using etch, you etch the bare metal but not over filler as it can burrow under it so, fill first, sand, then etch the remaining bare metal. Guide coat (ideally) to get filler smooth then more etch on bare metal if needed then prime and you know the rest.

If using epoxy primer, my understanding is you can fill/ prime either way round. It does seem that epoxy primer is the better route (but the 2k hardener bothers me).
Bill Bretherton

2 pack filler will have better adhesion properties to bare metal than etch primer. So do all your filler work first, then primer (etch primer or epoxy primer).
Chris Madge

Ok. So in summary,
Epoxy primer is likely to bond better than an etch primer, and is waterproof.
Epoxy can go on before filler as it adheres to bare metal, seals it from rusting, and filler will stick to it.
Or, filler can go on first, onto the bare metal, and then epoxy primer over it and adjoining sound paintwork to seal the lot before final paint prep.

I now see there are both 1k and 2k options for epoxy primers in rattle cans. Cans would suit me best for the spot repairs I am currently doing, but is there much to choose between 1k or 2k?
GuyW

From my days in Major contruction projects my understanding goes like this :-

Etch primer is still often deemed essential with metals such as aluminium where oxides form very quickly and priming adhesion is difficult and on galvanised surfaces that are not weathered etc but the primers are very thin.

However modern expoxies for steel are thicker, moisture resistant and some are classed as 'surface tolerant' and better suited where surface prep is not as good. Many contain varying quantities of materials including zinc / mica etc to assist corrosion resistance.

Generally (dangerous I know !) two packs have better durability/performance over single packs, however not all two packs are created equal !
The best are totally catalyst cured (no drying agents to evaporate and produce a slightly more porous film or solvent entrapement where any pooling may occur (not hopefully a problem with bodywork !
Also more likely that isocyanides and other nasties are in two packs.

I always checked out the manufacturers data sheets and their tech help services, especially for compatibility issues.


richard b

Taking this to the next stage. I am doing a running repair type refurb of my Austin Sprite. Last resprayed by a professional using 2k in 2011 but now looking pretty shabby. His job didnt last anything like as well as my own respray in 1994 using cellulose. Anyway, I want to do it myself again using cellulose this time.

I am progressing around the car attending to each rusted area welding in new steel as necessary and doing the first stages of filling and shaping, then sealing with the 2k epoxy primer. After that I will degrease the whole car and then go over it with a DA sander to key the sound paint and remove any damaged paint (some of it on the bonnet has cracked and crazed).

I think after that as I will have a patchwork of substrates it would be good to get a coat or two to seal all the varied surfaces. Would the 2k epoxy be good for that? I am likely to need to add further skim and guide coat it at that stage to get a good surface before cellulose colour coating. How does that sound?
GuyW

This thread was discussed between 19/07/2021 and 23/07/2021

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