What is the best stuff to use to coat and protect the floorboards under the car and under the carpet? I was thinking of using the black sealant stuff they use on house foundations. Putting it on with a brush, re-doing inside the doors, quarters and fenders too.

 

I used POR 15 then Fatmat and then EZ COOL heat deadener it all worked out nice.
I'm not too familiar with the house foundation stuff but if it's petroleum based it may start to melt or give off fumes.
Lizardskin looks like pretty neat stuff it is a heat and sound deadener.

https://classicoldsmobile.com/forums...-pict0070.html

Good luck

 

I also used POR and am very happy with the results, I first sandblasted then painted with POR 15 then applied the POR topcoat. The advantage of this type of treatment is that it will effectively stop rust or contain existing rust. The type of treatment they use on foundations will cover up the rust but will not chemically contain it. It will continue to rust underneath the coating. Dave E