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Planning on painting the steel purlins of a carport, that is peeling. I assume the primer and old paint are "oil based" alkyd. I think we will have it sandblasted. What primer should I use on the bare parts. Can I use an acrylic finish coat, if the paint that is left was oil?

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6 Answers 6

I would buy an auto-body primer and then use auto-body paint. You can get a really good look doing this especially if you spray.

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Once blasted. A single coat of red or grey oxide will make a good primer, they are not water proof though, so one or two coats of a good make gloss will seal it, spray to get a smother finish, if not blasting scrap what paint you can, wire brush the rust, and degrease. Then use hammerite, many customer I deal with ask us to use hammerite, but with clean steel, or shot blasted steel basic primers and gloss are easily as good, and a lot cheaper.

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Scrape it, wash it, fully prime it (any quality primer, but metal primer is best there), then paint two coats of a quality water-based paint. You'll be pleased with the result.

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You can apply a water-based finish over old alkyd paint as long as it is thoroughly de-glossed and clean. If it is steel, top quality grey or red-oxide metal primer (as Ben suggests) followed by top shelf exterior gloss paint. Contrary to what I often see/hear, using top quality paint and top quality applicators (brushes and rollers) makes a huge difference in the outcome.

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If you shoot the whole thing with primer, then there's little concern about acrylic-over-oil versus oil-over-acrylic. The primer does need to be chemically compatible with the new paint, of course, unless you shoot a layer of shellac before painting (shellac is compatible with nearly everything in the world).

Acrylic as a topcoat may last about as long as acrylic does on your house, but MAY not last that long if your local annual temperature swings are pretty extreme. Alkyd is very well-suited to steel, provided the surface is well primed; that's what park benches are shot with, and it's what I use for my Jeep restorations.

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You can use any type of exterior paint you want, so long as it's not galvanized steel which cannot be coated with oil-based paint.

Personally unfamiliar with acrylics, I had to defer to Engrav's Decorating's response to Acrylic paint over oil-based primer? from houzz.com:

We are a Benjamin Moore dealer. The rule of paint is this: You cannot put acrylic paint over oil paint, but you can put acrylic paint over oil primer.

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