I am done with welding in the floor and firewall patch panels. So, the car is on crib blocks so I can get under it. I have a combination of new welding, old paint in places, and surface rust in areas that I didn't have to patch. In some places I had to do a lap seam instead of a butt weld so those will require some seam sealer. I am thinking I will use POR 15 with a sponge brush to minimize dripping and, after it is cured, put seam sealer where needed then apply undercoating. I know the POR 15 can go right over the surface rust and the welds (after grinding and cleaning with a cup brush) but I am concerned with the old paint that is still seems to be solid. Do I just need to scuff that and then put the POR 15 over that?

 

They are lots of different opinions on use of POR15. My opinion and experience is that it really only sticks to heavily pitted rust. You can scuff the paint and apply it and it will look great, but hit it with a pressure washer and watch it flake off in big chunks. I realize you probably won't be pressure washing the underside of your car, but if a pressure washer can easily take it off it isn't really adhering.

I would use Rustoleum rusty metal primer and then topcoat with whatever. If you can put a good urethane clearcoat over all that you will be even better. Probably won't even need seam sealer.

I don't have much experience with "rubberized" undercoating and there are widely mixed and negative opinions on that, but my recent impression with those is that most are actually asphalt based tar products and would probably do a pretty good job of sealing out moisture if applied thick.

Of course nothing lasts forever though.

 

My only input is my experience with the Duplicolor rubberized undercoating. I sprayed the underside of the hump when I swapped my tank, and it delayed the project for a few days. It went on fine, seemed to stick well, but just wouldn't cure. After three days it was still very soft, I was just tired of waiting for it and put the tank back in anyway. Overall not a good experience. I'll be back under there for a rear end swap this summer, guess I'll see how it's held up.

 

I watched a video on the ol u tube by Repair Geek when I was researching the best rust preventative/sealer for my car hauler and POR 15 rated lower compared to other products. I have used POR 15 in the past on clean metal, I prepped it and did all the required steps it was ok. In the video I watched Mastercoat came in first with durability, but it also seems to be little more difficult to come by.

 

I talked to my frame guy today since he has done a lot of cars over his 30 years. He said he wouldn't use POR 15 since it likely would end up peeling from the old paint. He suggested just seam sealing the weld joints and then hitting the underside with undercoat. He specifically said not to use the typical auto stores rattle can stuff but to go to an autobody supply store (like Wesco in my area) to get commercial grade paintable undercoat. So, I am leaning towards cleaning the underside to get any grease and oil off, give the old paint a quick scuffing, wire brush the rust areas, hit it with rust converter, seam seal the joints, and then go with a commercial undercoat (like a 3M or similar) from the supplies store.

 

So, what is absolutely the best Rust Converter (not rust encapsulator) and Paintable Undercoating? I have been looking at Corroseal and Gempler's rust converters and SEM Paintable Undercoating right now.

Thanks