I actually have done lots of research on this topic but haven't found my exact situation in my searches. I'm resto-modding my 67 and the previous person had undercoated right over rust and grease. I'm taking it down (just the frame, wheel wells, engine bay etc....no body panels) to bare metal and want to prime it and then apply some sort of lizard skin (or something similar) product over the primer. My problem is that I am renting this house and cannot turn it into a paint shop. I don't own a compressor or any real knowledge of painting vehicles. I'll be needing to do this all from spray cans. I've been told to use a self etch primer over the clean metal (after prepping) and then pretty much confused after that. Do I need to coat it with something else first (paint or sealer) and THEN the undercoating product? Do I scrape the etch primer off and start with something else? I've just begun to strip the car so I'm in the initial stages. I'll have to do this in sections due to the massive buildup of grease, undercoating and rust. ANY advice y'all have for me would be beyond appreciated!! Thanks so much for your time and knowledge!!!
Ok, so no compressor.
I understand the dirt and grease! My red car was packed with red oklahoma dirt. I laid down on my back for an entire day underneath it just washing all the dirt and grease out.
What I did with my red car is to use POR15 over the bare metal. It calls itself a rust encapsulator. It just seals itself up completely, not allowing any more moisture onto the metal. I coated the entire undercarriage with it, just as a precaution.
If you have any rust to deal with, first off, you have to determine how bad it is. (has it rotted through the panel?) if it hasn't, it needs to be neutralized with something like eastwoods products. You'll have to sand off all the scaling first. You may need to retreat it if the rust hasn't been completely neutralized from the first treatment. If it has rotted through, you should at least cut it out and weld in some new patch panels.
Can you take the vehicle to be sandblasted? That would completely remove the rust. It's not very expensive to have this done either.
Try not to just paint over the rust with the POR 15. It says you can do that though. I myself wouldn't.
I applied the POR15 over my entire undercarriage and even on the floor inside the cabin. I used a small paint roller and some cheap hf paint brushes for this. The POR 15 levels it self out real nicely, no brush streaks or anything.
After letting that dry for a couple days, i scuffed the POR 15 with red scotchbrites and sprayed 3M's undercoating over the entire undercarriage. I sprayed two coats. I just used the rattle cans that 3M offers. I think i used right at 6 cans of the undercoating.
The undercoating will also help with being a heat and sound barrier.
Do both things though, it'll last far longer than just one of them by itself.
Be sure to use a drop cloth when applying both of these products. Make sure you clean up the splatters from the POR 15, once it dries, it gets permanent. Even on your skin.
Here's some pictures after i coated the undercarriage and one of the interior. You may see where i had to replace the rear floor pan areas and some of the other patch work i did because of rot, plus a homegrown mini-tubbing.