Long story short, my heater core went and rotted the passenger side floor pan right by the drain plug. Getting under the car and taking a look around, the driver's side is starting to rot through as well, I am going to weld in new ones from a donor car. I decided to refinish the whole underside and I've cleaned up 90% of the rust under the car so the car is about 50-50 bare metal and paint after weeks of wire-wheeling. My plan is to clean with dawn soap, degrease, and then prep with an appropriate solution depending on paint choice. My original plan was to use POR-15 since the rust was so bad. Since I've cleaned most of it up, I'm hesitant to use POR-15 as I've heard it adheres poorly to bare metal or other paints. POR-15's data sheet says it will stick to both, but over paint it does not inhibit rust obviously, so it seems unnecessary to use POR-15. Rustoleum seems to lack the strength to hold up long term, most people say they need to do touch ups every year which I would prefer not to do.

My question is this: how would YOU guys refinish the underside? I plan to have the car for many years to come, and I would only like to do this once given the amount of work I've put in to clean the car up. I've got a few pictures below that show the general condition of the underside, and how much I've stripped it. All advice and info is most appreciated. Thanks in advance.


Gas tank area


"new" passenger's pan

 

I did a very similar thing on my car when I had it torn down to a bare shell. After months of stripping undercoating, wire wheeling and sandblasting, the underside was in similar condition to yours now. I used a phosphoric acid metal etch/rust neutralizer to take care of the rust in the pores of the metal that I couldn't get with the wire wheel or sandblaster. After letting the acid sit for about 1/2 hour or so, neutralize with plain water and then dry quickly with compressed air, then clean with wax and grease remover. For coatings, I used R-M Diamont 2 part Epoxy primer followed by R-M Limco 2-part urethane single stage paint in semi-gloss black. It's going to be more expensive than a rattle can job since it's professional level products, but it's significantly more durable.

 

Epoxy primer and topcoat of choice if desired.

 

Wow, that looks like it came out great! I was hoping to get away with someone that was able to be brushed on, but seeing the results of people who have sprayed the underside, it's worth the extra masking. Will the acid etch solution harm the existing paint? I don't want to compromise the paint that is going to be primered over, and it seems like unnecessary effort to get the underside down to 100% bare metal.

 

Use the acid etch on the bare metal areas, but it's ok if it gets on the adjacent paint that's going to get painted over anyway. As long as it's only on there for a half hour or so and then washed off, it won't lift the old paint.

Body shop supplies and a paint gun will certainly provide a nicer end product, but it is more expensive, need to mask things off, need to protect your garage from overspray, need a paint gun and compressor, etc. If you are looking for something less expensive and brush on, you could look at something like Eastwood Chassis Black.

 

I have a full paint setup from when I painted the exterior of the car, I just really want to make sure I keep the overspray down so rattle cans would be handy. I've had good luck using Martin Senour's new 2k epoxy primer in rattle cans on wheels, I think this would be another good application but I will ask the paint store.

First step is acid etch and clean, thank you for the advice!!

 

The 2K rattle cans are pretty good, You will use a lot of them though.

 

Good point, that probably isn't cost-effective. Looks like I'll be breaking out the paint gun and spraying it. How much did your underside take to coat?

 

About 1/2 gallon of primer and about the same for the topcoat.

 

 

Wow, the dry ice cleaning is impressive. abrasive enough but not damaging. The protective wax seems do-able, but I'm going to go the epoxy route since I'm already at bare metal. Half gallon of each it is!

 

Lizzard Skin is what what i would recommend for an under coating.