Paint question

I've got a question for those of you with painting experience on old iron. We have a flat car at our rail road museum that was set up as an open air car about 6 years ago. The car frame was sandblasted and primed but never final coat painted. I don't know what the primer was but now there is rust coming through the primer. My question is can a rust killer be applied over the primer and then repaint or does the primer need to be stripped off and start over again. What happens if we just reprime and put on a final coat of paint? I have 12 of the old tractors but not one with a new paint job so painting is not my area of expertise.

I would sandblast it off start over. Since you dont know what primer was used it could cause problems when you apply your paint. And if there is rust now you might have problems with rust later.

Unfortunately I think that the rust restoration products dont do well with painted surfaces . Be nice if the did , but moisture is getting under the primer ,it will only continue . Wasting your time , restoring efforts .Reblasting the frame is the best suggestion . Then an acid etch product is worth doing to before priming . Sorry but fact is corrosion starts immediately after exposed to the air .

Primer is (in most cases)porous. This is to create a easily adhered to texture that is 3D. The real paint then penetrates and attaches as it hardens. I would probably recommend painting one area with rust inhibitor, then painting as desired. If it shows rust in a year, make decisions about blasting and repainting then. If it must be archive quality, sand blast and refinish now. Jim

If you paint over nasty, then nasty will just be laying there underneath a thin layer of paint.

I know they make rustoleum paint and all this and that, but you still can't expect it to perform miracles.

Painting over an old coating of weathered unpainted primer, probably wouldn't be much different than painting over a layer of dirt or grease.

For what it's worth, rustoleum paint is probably intended to be sprayed on a bare surface that has a little rust. Not over a caked on rusty layer of primer with all kinds of cancer and deterioration going on underneath.

You could probably spray it as it is with paint and have it look good for a minute and from a distance, but that'd just be a quick fix to an eye sore. Not really what you want for a muesem piece, is it?

It really should be stripped back down. Truth is, if the primer wasn't going to be painted over right a way, it really should of just not been put on. In that case, a guy should of just went with straight paint, and forgot the primer. If you were sitting in that boat right now, you could probably just sand a little to achieve a scored surface, and have at it with the paint again.

A primer is a little bit like a sponge. It's characteristics are a little different than paint.

Without a picture, it's hard to give an opinion, but here goes. If the rust is coming through every where, re-do. If it is bleeding through in isolated spots, sand and re-prime those spots and paint. I doubt after 6 years of aging outdoors, you will get any reaction between the primer and the new paint. Trying a small hidden spot wouldn't hurt. For me, Rustoleum would be the last paint I would put on something that is going to spend it's life outdoors. Better off with an industrial enamel w/hardener.

I wouldn't waste time and money by trying to salvage the primer on it now. I would blast and then go back with an industrial paint. 3m makes a really nice DTM (direct to metal) epoxy paint that takes the outdoors really well. Find a 3m paint supplier and see what they can get for you. Won't be cheap ($150/gal), but worth every penny.

Rustoleum has a fanastic brand name ..... for marketing. Up here a similar brand is Tremclad, and several others too including Rustoleum, all called 'Rust Paint'. You buy them at the corner store everywhere. Personally I think they are just another solvent based machinery enamel (water based versions are available too) and a cheap version of a good commercial/industrial machinery equipment paint. I guess I strayed a bit but I can't help with your question.

If it were me, I wire wheel it then brush on Por15. Then paint over it after a couple of weeks. I did that on my gooseneck in 09 and the rust hasn't came back yet.

As I was reading your post I immediately felt the car needed to be sandblasted again. Primer is very porous and allows water to go through it. Now you have the metal rusting under the primer. The primer will soon start coming off and if you paint over it it will take off the new paint with it. Had it been primed with an epoxy primer there may have been a chance of salvaging the paint job but rust wouldn't be coming through epoxy primer. Really if you do choose to start over I would recommend using an epoxy primer. Once metal starts rusting it's difficult to stop and epoxy will really help putting an end to rust. Even on a car with any other primer if you get a scratch down to the metal the rust will spread outwards from the scratch under the paint until you have a much larger spot. With epoxy primer if you get a scratch down to the metal the rust will be kept isolated to the scratch.