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?
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.
It is fairly unusual for bare steel (with paint) to peel, it rather will develop rust spots and blister off. The typical scenario is that your posts are galvanized and the galvanizing on the post is "rusting" which means its forming a zinc oxide layer (which is powdery) and pushing the paint off.
The fact that you have not mentioned rust will also confirm that the purlins are galvanized.
If you are going to sand blast, you will probably remove some of the galvanising coat, so watch out there. Does the purlins have lots of contours?
If you are dealing with raw metal then I would scrape off the old coat with a scraper (or sandblast) then inspect for rust, if you have rust then you will need to give a light sanding followed by a rust inhibiting red oxide primer or any other Quality water or acrylic based primer.
The thing with choosing a top-coat paint is largely a factor of how you want the finish to look. Typically the oil based enamels are quite glossy and oily, if you are looking for a nice sheen, then go for water or acrylic based. I find that Acrylic paint is great to paint with, but you must give two coats of primer first since the paint is not good at creating a preventative sealing barrier (it seems more porous)
Yes you can paint acrylic on oil, It should not be a problem, the only risk is when there are contrasting chemicals and they react, so its always prudent to paint a small sample before-hand to be sure.
If you are dealing with galvanizing, the issue is a lot more complicated, you will essentially need to get the old paint off (just so you don't see the uneven paint surface) If you sand blast, then you will have "fresh" galvanizing which means that it should actually "rust" before you paint (i.e. leave it for a month) then Paint with a good quality etch primer followed by a compatible top-coat, (etch primers are the nasty guys that actually burn into the metal to create a nice grip) you will need to pair this paint when you buy it since different paints and manufacturers are more or less suited.
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.
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.
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.
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.
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.