I've got some paintwork to patch up behind the SLD rail. About a quarter of the area behind the rail in all, say 30 square inches in two inch strips. I've taken the rail off and cleaned up, and its down the the galvanising generally and through to the steel in a few places, with minor rusting.

I've read that acid etch primers are good for galvanised bodywork, or bare metal, but can 'eat' under other layers. Would you recommend a spray can of acid etch primer for this job, and if so, an acryllic or other fill primer over the etch primer before topcoat and lacquer, or something else?

I've also got several patches with the common topcoat flaking problem, so could I use etch primer over the existing primer to sort those patches too?

Many thanks, Dan

My experiences with etch primer are not good. The greenish primer Fiat uses on the 2007 (and probably later) models forms blisters within a few minutes. The same happens when acrylic paint is sprayed on the primer unless the paint is applied in very thin coats 10-15 minutes between layers.

I guess Fiat uses only water based paint products that are very sensitive to solvents.

Etch primer has phosphoric acid which is of no benefit over anything except metal. Some acid is trapped and can lead to osmotic blistering.

Hi Dan, Hi, I've never used etching primer on galvanised metal only on aluminum so cant say how it would work on it.
However, it is great on ally, I've only used it using spray gun, if using pray can you would have to use thin coats and build it up slowly.
Spray cans with the paint having to be so thin can affect the surrounding paint.
If you have flaking paint the only answer is to sand it all back with wet & dry paper at say 400 grit.
I would not try spraying etch over your topcoat.
After using etch just use ordinary cellulose primer and build the coats as normal.
The Fiat cars I have worked on have had the basecoat & laquer system used on them, but that was a long time ago and may be different now lol.

Thanks all. I went ahead and used the etch primer, three thin coats, and I can't see any blistering where it overlaps the base (greenish) primer. Hot sunny day here so maybe that helped.

The etch primer I used was what I could get hold of in a hurry - grey from Halfords, and I am hoping that it will cover okay with 2 or 3 thin sprays of white topcoat tomorrow, but maybe I ought to pick up some white cellulose or maybe acrylic primer as an intermediate coat?

Etch primer is totally porous it will need to be well covered to keep the tinworm out. ypu can get 2k primer in a can now there's bulb on the bottom to release the hardener. that's tge best way to keep rust at bay.