Cleaning the underside up. I have to body on a rotisserie. Light surface rust only. Would you POR15 the bottom and top coat or use rubberized undercoating? Car won't see salt and will only see rain if I am caught out in it unexpectedly.

 

"POR15 the bottom and then top coat" The POR will ahere best to a clean aged finish even with with light surface rust. The metal must have some bite, so a rough surface is best.

 

I take no chances with my builds. Cars to be driven get POR-15 (or Master Series, which I like better) first, 2 coats 12 hrs apart. When the second coat is still slightly tacky, I put on rubberized undercoat, or better yet, bedliner. It's smoother, tougher, looks nicer, and doesn't absorb oil and grease, easy to clean. Use one that comes with a hardener, not a single component one.