I have some light rust on a primed roof surface (see attached pic). What would be the best way to get rid of it? I'm going to put plastidip on it until I can affort a vinyl roof.

strip all of your primer off then use a high quality epoxy primer, like PPG DP epoxy primer.

But you have to get rid of or neutralize the rust FIRST.

Blast, treat and recoat, then seal.

Couldn't I use some of that picklex 20 rust converter or something similar? I should say I'm in an apartment building and have limited tools, no air tools.

The rust has gotten under the primer... hopefully the primer is thin! With limited tools, I'd suggest hand sand it thoroughly. 120 grit would be OK. Then shoot it with red Rustoleum rust stop spray bomb primer.

If you want to smooth it, shoot it with a couple of coats of Rustoleum auto primer and sand with 220 when well dry. It may need another coat of primer at that point. Hit it again with primer and sand lightly with 220 before the finish coat.

i believe you need to sand the area to bare metal first, then use a wire wheel to clean out the remaining pits. after that, neutralize the pits with something like acid [i can't recall the exact one]. this will kill the remaining rust as well as really clean the pits. next, neutralize the acid. then, top coat with the primer and or surfacer of your choice.

Are you sure the rust is from underneath? The mechanic who worked on the car grinded down the whole roof to bare metal and put this primer on just last year.

Do you want to risk it? It'd be bad to get a top coat on it only to have it come through. Grinder dust is like rail dust (get on new cars during rail transport) once it rust on the paint it is next to impossibly to remove it and stop it remove rusting back and that is on new hard base coat clear coat, primer doesn't have a chance. Either do it right or leave it alone until you can do it right. Most primers don't seal out moisture so whatever work you do you need to seal it with either a quality sealer or even a cheap top coat otherwise you will be in the same situation in a year or so.

Primer is porous and will not seal out moisture, so water or even humidity will soak through the primer and start the rust . What you see is where it is the worst , if you look closer you will see that it is all over the primed area. The primer will have to be removed down to bare metal. The rust will have to be removed probably chemically , I wouldn't recommend blasting on a roof. Then re-prime and paint

Do you mean rust oleum stops rust? Why red?

Rustoleum primer seals up and helps stop or slow rust. I have found it to be quite effective. The red primer in a paint can or its spray bomb cousin is specifically formulated to stop rust... more so than other Rustoleum products used as primer. Clean well and remove any loose paint or loose rust then shoot it. Rustoleum primer plus a top coat of some type should hold it for several years at least. Huge improvement over what you have and doable without a shop or big budget.

Is this the one you are refering to?
https://www.rustoleum.com/product-catalog/consumer-brands/stops-rust/automotive-primer-spray

This. Spot on. No alternative.
Robert

What specific chemical would you use to treat the rust?

He's right. I'd do this and follow up the sanding to bare metal with Rust-ese (it's organic), then wax and oil remover, then etching primer, scuff it and shoot high-fill primer in case you want to paint and not vinyl the thing. Then paint. and I'd pull the bright trim too! Chances are they didn't shoot paint under the trim and this is where rust loves to grow. Rust is not a game, son.

This one is best for the first coat or two: "Rusty Metal Primer. The automotive primer in red or grey over the rusty metal primer would be optional to help smooth it. The auto primer sands better.

If you use Rustoleum best to observe label directions on re-coat time. Usually it will say "re-coat within 2 hours or wait 48 hours". I usually apply the primer in two coats about 10 minutes apart, wait a half hour or so to let it set up then apply the finish coat. If you re-coat between 2 hours and 48 hours it may wrinkle or lift.


https://www.rustoleum.com/product-catalog/consumer-brands/stops-rust/rusty-metal-primer-spray/

Hmmm, let's See ... under the primer you have metal. On top of the primer you have ... nothing. Yeah, I`m pretty sure its from underneath !!

I sanded down a small area that had rust on the primer. The rust disappears half way through the primer, there's no rust on the metal. But will sand down anyway to be sure.

So basically I would do the following:

1. Sand down to bare metal
2. Clean with wax and oil remover
3. Spray with two coats 10 minutes apart with this product
https://www.rustoleum.com/product-catalog/consumer-brands/stops-rust/rusty-metal-primer-spray/

4. After 30 minutes spray this product. How many coats?
https://www.rustoleum.com/product-catalog/consumer-brands/stops-rust/automotive-primer-spray

How many passes with the spray can, would you consider one coat?