roof rust "paint?"

the sons roof is fairly rusty. he spent the day with a DA sander but the metal is still "brown" and not cleaning up and shiny like fresh metal. We are hoping to just paint it with something until he can afford a paint job, which would likely require filler up there to smooth it out. With winter coming, we are just trying to prevent it from getting worse.

Is there any product that should be applied to the sanded metal to stop any rusting at this point prior to spraying with a primer?

You could use rustoleum rusty metal primer, it is meant to be painted over rust and won't work well on clean metal,top coat with regular rustoleum. But your going to need to strip all the rustoleum products back off when you decide to do the job right.

I’m not a body & paint guy, but could you use POR-15 and then paint over it later? It’s used a lot to prevent rust on the chassis or in the engine compartment, but I don't know if you can use it under a body paint job.

Epoxy or DTM primer. Then decide whether to top coat it or leave as is. Epoxy should be weatherproof for a “winter” if appearance doesn’t matter. Other primers, especially polyester (typical filler primer) is more porous and not suited for long term moisture exposure.
If you’ve done all the basic prep, it makes sense to spend a few bucks more on something you won’t need to remove completely later when doing the actual bodywork.

Good points from Todd, epoxy would a fine choice.

It is not an epoxy, but Tamco’s 5311 is pretty resistant to the elements. I replaced my trailers fenders this time last year, primed them with 5311, it has sat outside for a year, snow, rain, wind, sun, they look the same as they did the day I sprayed them.

Thought about Por 15 as well but I think removing it later on for finishing might be a problem. That stuff is like iron when dry.

How much of the vehicle is rusty? It might save you a lot of time to prepare it properly now rather than undoing a seal coat. Does anyone in your area do slurry blasting? I used that process on a bed last year. No metal warping and quick.

Try an oxidized area with naval jelly, cover with plastic wrap for a day or so, preferably indoors. I’ve had good success with it to remove rust. After that, epoxy primer like most recommend. Wear proper safety protection when using.

Ricko1966 said:

You could use rustoleum rusty metal primer, it is meant to be painted over rust and won't work well on clean metal,top coat with regular rustoleum. But your going to need to strip all the rustoleum products back off when you decide to do the job right.

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I used Rustoleum rusty metal primer on a pickup box trailer a few years back and it holding up well. I only used the rusty metal primer on the bare metal that was rusty. The whole thing was rattle canned. It's a trailer, so didn't car about show car finish.

Just note, it REALLY sticks! I was trying to sand on some pipe slide rails for my jeep that I had used that primer on, and the primer really didn't want to come off.

POR 15 is not UV friendly. It will be very hard to remove later.

Muriatic acid dissolves rust.

Like @fast99 says - you will want to get the roof wet "sand" / media blasted to clean out the rust to bare metal. whether you do this first, or come back to it later, I would invest in an airless spray gun and hit it first with a cold galvanising ( zinc ) primer. This should prevent further rust. A high build primer should be next, to fill and smooth the surface. Then seal everything with a straight enamel gloss top ( colour ) coat. The gloss coat will seal the primer coats to prevent moisture getting to the surface.
If you can clean the metal back first, you will just be able to wet sand back / re-spray to get a good finish later., if needed. If you primer and paint it without cleaning the metal, you will need to get everything blasted back to bare metal when you are ready, and then build up from there.
I wouldn't use POR or any of the rust treatment paints, as they will be a real pain to try and remove later for a proper finish.