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Tue Jun 04, 2013 10:09 am

Hey all,

My bus has a weird paint issue.

Based on what I know about the P.O. that preceded the folks who sold it to me, here's what I think happened.

The P.O. stripped the paint, but then got too sick to finish the job so the car sat for many years and got surface rust. After that, rather than sand it all down to bare metal, the next owner applied a skim coat of bondo and then painted it. My dilemma is that the skim coat application was done pretty well.

The result? Before I got the car, there were rust bubbles that migrated up, so I have some rust spots that are pretty bad. The only way to fix them is to sand down to bare metal. Most of the bus is "stable", meaning that the rust is either sealed off or non-existant. The inside of the bus is almost completely rust free and looks really good.

The way I want to tackle this is to sand down to bare metal, cutting through the bondo skim coat. Then apply some kind of metal stabilizer, or even something like rustoleum, to seal it. Then apply a skim coat over that to even it up. Then paint.

My question is, what products should I use? The skim coat of bondo was done very well and the bus looks straight. If I can fix the rust bubbles properly and put on new paint, it should look great. My concern is paint compatability.

Recommendations?

Steve

Tue Jun 04, 2013 10:46 am

Even sanding down to bare metal on a rusty area, you will still have pitting that you need to get the rust out of. So worst case, before you encapsulate the rust to seal it off from oxygen and further reaction, you'd want to neutralize it.

A phosphoric acid product (Ospho or similar) will chemically convert iron oxide (rust=reactive) to iron phosphate (a chemically stable compound=nonreactive). The plus with this is it acts like a primer at the atomic level, sealing the metal from reacting with oxygen.

One way besides grinding to treat those pitted rust areas if they're small, is by dabbing on muratic acid (used for cleaning pools and tile). It will eat the rust and leave shiny unprotected metal. You'll have to flush with lots of water to stop the reaction and remove any remaining acid. You can then let that flash rust and convert that with phosphoric acid and have your first layer of primer.

Another thought before you fill those pitted areas with filler, is to paint with a good primer (like epoxy or a urethane Master Series) and then apply the filler on top of that protection.

This is the way I have been preping my car as I work on sections of it at a time.

Good Luck...

Tue Jun 04, 2013 1:12 pm

Thanks for the info. So the Ospho can't reach into the pitting??? Here's a follow up question.

After I put on the bondo, can I spray it with a "spray bomb" type of primer if I will primer the entire bus properly? Or, can I cover the bondo with a "spray bomb" rustoleum paint if I primer it properly?

I guess the question I'm asking is whether a good final coat of builder primer shot from a spray gun will be incompatible with my patchwork that I covered with something that came out of a can with compressed air?

TIA,

Steve

Wed Jun 05, 2013 5:49 am

Westy Steve wrote: Thanks for the info. So the Ospho can't reach into the pitting??? Here's a follow up question.

After I put on the bondo, can I spray it with a "spray bomb" type of primer if I will primer the entire bus properly? Or, can I cover the bondo with a "spray bomb" rustoleum paint if I primer it properly?

I guess the question I'm asking is whether a good final coat of builder primer shot from a spray gun will be incompatible with my patchwork that I covered with something that came out of a can with compressed air?

TIA,

Steve
The Ospho or phosphoric acid can convert the top layer of rust in the pit to a stable compound, but you'll still have that pocket of rust underneath it. Theoretically it shouldn't rust any more with that area sealed off from oxygen, but I would rather get all the rust out before I go to all the trouble and expense to paint it. But that's just me.

I like to seal the bondo area before laying down any primer and paint so the bondo doesn't act like a sponge and suck the primer into it. Again, that's just what I prefer to do. No way of knowing if the coat of good build primer you use will be compatable with the spray bomb stuff you use.

I've been using Master Series products, specifically the Sliver primer on my car, and can't say enough about it. I've used it on bare steel and over filler to seal the area before high build primer. I've had to go back and rework some areas because I didn't do them quite the way I wanted to, and that Silver primer is a bitch to get off! It bonds well to everything including your skin!

Wed Jun 05, 2013 8:13 am

Westy, I am jumping in with "no!" do not use any spray bomb primers on your car. I am restoring a 60 beetle, and it has taken me over a year to get my body work done. Wherever I sanded through to metal or made a filler repair, I used SEM etch primer (from a can, on exposed steel) and Rusto auto primer (from a can, on filler and metal). My thinking was that that I could protect the metal from rusting, see my panels in one color after a filler repair, and then use the primer as a guide coat during final sanding and paint prep. Big mistake! The canned primer is crappy to remove, and it ALL has to come off due to compatibility issues. I have read too many nightmares about new paint bubbling up over it. So, I have spent the last month sanding and dousing the car in laqcuer thinner to remove it, when I could have had it painted by now.

I would rather be sanding bare metal areas then trying to remove this stuff.

If you absolutely feel you must put something over the repair area, make sure you put can primer and paint on flat surfaces that you can sand easily, and plan to remove it ALL. And keepin mind, sanding this off will possibly cause you to reshape your filler repair, and you may end up having to re skim and sand.

Personally, I will never use it again.

Wed Jun 05, 2013 9:03 am

No, the spray bomb primer does not All have to come off. Rough it up and spray your epoxy/paint/clear when it is All straight & it will not bubble/peel!

Wed Jun 05, 2013 10:37 am

This reminds me of a movie where the guys says "you won't know who to trust".

OK, so to play it very conservative, we seem to be dealing in blanket statements. Any specific primers someone can recommend, or disavow?

Bear in mind we are dealing with rust spots. Here's a really bad picture and my bus is dirty from storage and blowing oil, but you can see what I'm up against:

Here's what I have so far:

1. sand and grind off rust.
2. brush on Muratic acid followed by lots of rinsing.
3. Allow it to flash rust and then ospho it.
4. (optional)...spray "extend" or a rust converter paint, followed by bondo.
5. Cover the bondo with a thin coat of something that won't react with a good quality build primer...or perhaps something that will sand off easily later before final build primer coat.

Does this sound right? Which step numbers are wrong.

Thanks,

Steve

Wed Jun 05, 2013 11:58 am

Well you're right, there are a lot of blanket statements being thrown around. Step two is optional, but will guarantee you get rid of the rust. It will also strip off all the natural oils of the metal, hence the ease that it will flash rust after rinsing. For step 4 you could substitute a good epoxy primer or equivalent. Everybody has their own method of doing things with different products, so there's bound to be conflicting info.

One idea I have a problem with is the idea that a primer that comes in a can is not of sufficient quality to paint over without having compatibility issues. I can't imagine the makers of SEM products, which are some pretty decent products IMO, don't envision top coat paints going over them. In any case, I'll let you know in a few weeks when I start painting a Urethane base coat over SEM high build primer.

Wed Jun 05, 2013 4:09 pm

you guys kill me......

sand off the heavies, get it down to a point where you hit good clean steel (probably 3-4 inches around the rusty area) get a spot blaster and carve out the rust with a sand blaster (one of the hand jobbies)

epoxy, prime, paint, done. if it is pitted deep after the epoxy, use some filler, sand re-epoxy to bury the filler.

no need to get all tin foil hat crazy with goofy chemicals that can stay with the steel and blister back later. stop overthinking it

Wed Jun 05, 2013 9:37 pm

I do trust what mike says and hope he is right. I have scoured this site and others, probably too much, and some of the posts about using auto primer and paint over spray can primer have scared the crap out of me.

A conservative approach is always safest.

Tue Jun 11, 2013 8:48 am

Car was painted last Saturday with PPG concept ss. I know I had one area covered in SEM spray can metal etch gray; the engine louver area... I could not sand that area, only blow it out with air. It was the primed with an epoxy sealer and baked a few hours. Then paint and bake overnight. Everything looks beautiful, with no adherence issues that I see. I know there were a few places I could not do a thorough job of removal on, and those places look fine.

I don't think I would leave a ton of it on the car, but unless time tells otherwise, the nightmare I was having was unfounded.