Rust band-aid: what did I do wrong?

[Carpe Discus]

I've got two spots that had rust: above my back hatch and my roof ribs. I used one of those sponge-like blue grinding wheels to hit the rust on the roof (unfortunately getting some metal too), and then painted with several coats of rustoleum enamel. I'm thinking I shouldn't have been so aggressive here but nothing else was working. Problem is, the rust is back like I'd never attacked it--maybe worse.

The spot on the back hatch, I had a patch welded in that I sanded down, even hit with phosphoric acid, and covered with several coats of a primer/sealer. It's now showing rust too.

What am I doing wrong? Why can't I get rid of the rust? It doesn't seem like rocket science but there's clearly something I'm not doing right.

 

[Southeast Overland]

You didn't remove all the rust before you painted.

You probably have rust hiding in a seam that is coming back out, or you didn't remove all the rust from the patch area, or there is rust coming through the metal from the other side.

Primer is porous and will allow the rust to come back.

Rustoleum is OK but will allow the rust to come back.

You need to get more of the rust off and use a product like POR15 or RustBullet.

 

[Carpe Discus]

I was under the impression that POR15/RustBullet was more for underbody surfaces and less for stuff that you were going to actually have a good finish on. That is, I thought it went on with a rougher finish that's pretty hard to smooth out since it clogs the sandpaper. Is this true?

As for the primer, I thought I was using a non-porous primer but I suppose I could have been mistaken. I wouldn't think the rustoleum was porous either, but clearly the rust is back so something's clearly wrong.

 

[Southeast Overland]

I've used POR15 and it self leveled VERY nicely. You are supposed to use the company's special primer on top of the POR15 before you top coat with your final color coat.

I haven't used RustBullet myself but a friend has. It went on easier (less prep) and leveled pretty nicely but I didn't look that close.

I don't recall the POR15 clogging sandpaper but I also didn't sand it much.

Using POR15 or RustBullet, even if it doesn't leave a perfect surface for top coating, will be MUCH better than having the rust continue to take over.

 

[Dieseler]

All primers are porous and let moisture in eventually.
Rustoleum is a bandaide solution short term not long term,rust can be almost stopped if moisture and air cannot reach the effected area.
POR15 or Rustbullet will work for long term if prepped and applied properly.
Pics may help to see what the problem might be,lets see both sides of the area in question it may be the other side that is causing the problem, is this a seam or a flat area of steel.

 

[Luke111]

Don't foget to hit it with some rust converter

 

[Gusb]

My experience is limited, but I've had pretty good luck with some Rustoleum band-aid work on my old pickup. It's been outside (all winter) since my 40 took over the garage last summer, and the Rustoleum "surface rust" repairs I did a few years ago seem to be holding up fine. I've also used a bit of POR15 on the truck which has done well. I think prep/cleanliness is always the big issue no matter what the final paint process.

If you've got rotted through areas, the only solution (IMO) is to cut out the corrosion/bad stuff and weld in new material. POR15 can do a good job of sealing in most cases, but with lots of deep rust where moisture still has access, who knows how long it will last.

For surface rust (again, IMO):
1) Strip the old paint (chemical paint remover, etc.)
2) Clean with pro-type degreaser stuff
3) Sand/grind/blast (in moderation) to remove as much rust as reasonable
4) Clean
5) Treat with "Metal Ready" (from the POR15 people) - make sure to rinse (with water) and dry (and keep clean before you paint). This stuff seems to really help and it's not too nasty to use. Put a little on an even new piece of sheet metal and rinse and you can see it's doing something. It also seems to react with/seal whatever surface rust is left. In general it seems to even help paint like Rustoleum last. Just make sure you do the proper MR water (even though it sounds weird) rinse, etc. You need to neutralize the rust treatment or you're just creating more paint problems.
6) Keep it clean, use gloves, etc. before you paint.