My car ('03 Maxima) has a rust stain emanating from a joint between a plastic panel and a painted steel body panel. The visible painted area is not yet rusting; the rusting metal is presumably in a hidden area on the mating surface, and the stains are being carried out into the visible area by rain water. I’d like to stop this rust from spreading, but since this is a non-visible area I don’t want to go to all the trouble of sanding/priming/painting the rusty area unless there’s really no other way.
I’d like to remove the plastic panel to expose the rusting area, flake off any bubbled paint and rust flakes, and then slather the bare metal with silicone RTV. Will this stop the rust spot from spreading, or do I need to go all the way with priming and paint?
The way the pros do it is they remove as much of the rust as they can, then they either dissolve all of the remaining rust of convert the rust (Fe2O3) into black oxide (Fe2O4). This will stop the rust from spreading, and once it is done then you can cover up the metal with something that will inhibit rust formation.
If you don’t handle the rust underneath it will still manage to form new rust even though you’ve covered it with RTV.
I’ve never been able to find the chemicals that the auto shop guys use but then I haven’t really looked very hard either.
ETA: Disclaimer: I am not an auto body professional. This is just my understanding of how they do it.
The best way to stop rust from spreading is to remove the rust, replace whatever became the rust and THEN seal it. If you seal over rust it will just rust behind the seal.
I would remove the plastic that is hiding the rusty patch and assess the situation. It may be that it’s a bit of surface corrosion that can be sanded away and then sealed over, if it’s not a visible area then there’s no need to paint it.
If you’re unfortunate, by the time you remove the rust there may be a hole in the metal panel. The size and location of this possible hole will determine what action you would need to take next, whether simply sealing the bare metal, filling the hole with fiberglass, or even welding a patch over if it was particularly bad.
When it comes to sealing the job, there are far better things you can use than silicone, that’s primarily used for sealing the joint between two surfaces, rather than to keep the weather off. I’d go for a car body sealant like waxoyl, which has corrosion inhibitors to help prevent a repeat occurrence, but be careful as it can be messy if you’re not.
I don’t know the chemistry of it, but Rustoleum makes a rust-converting primer that does what you describe.
I’ve used it with good results on cast iron stuff that was so “active” that you could sand it smooth and shiny and almost watch it turn brown before your eyes.
Aside from that, yes, you do need to remove as much of the existing rust as possible or it will continue rusting away behind the scenes, so to speak. Pulling off the plastic panel, sanding the surface and hitting it with converting primer and then a topcoat of whatever color should be enough to stop the OP’s issue, as long as the rust hasn’t pentrated through the metal and gone…
<ominously> To The Other Side! </ominous>
I popped in to say POR 15. It loves rusty stuff. I have used it for over a decade. Wear gloves. Your hands will be stained for 4-5 days if you don’t. Ask me how I know this. It is crazy hard, with a plastic like finish. It is UV sensitive, so top coat it if exposed (read directions). It will outlive you.