to answer the first question....
I have used Aluminum Mobile Home Roof Coating on floors,bumpers,and the roof of my van..it lasts longer than paint,hardens enough so it does not stay sticky or tacky,and it helps keep rust to a minimum..its cheap,about 12 bucks a gallon,35 or so for a 5 gallon pail..a little goes a long way when brushing it,a gallon did my vans roof,bumpers,and ana a home heating oil tank ,still had some left over..
It does slowly wash off on roofs,but it takes years--I did my van's roof in '99,and its just starting to show the paint underneath it...but I have noticed no "wear" on it as far as the bumpers and floor on my van..I'd paint the whole undercarriage with it if it wasn't surface rusty already..it's not Herculiner by any means,but its better than paint or nothing...
The black asphalt tar roof and foundation coating makes a good undercoating or floor protectant,but most of that stuff never dries completely--Labrea Tar Pit is a good description of what that stuff is like..but I used it to coat a frame once,and it took ten years for our salty roads to finally peel it off!..and I got "tarred" every time I worked under that truck too..
There are other roof coatings available that are rubber,like "Snow Roof" or "Cool Roof" sold at RV centers that coat the steel with a rubber like stuff that is fairly durable too...its more costly than the asphalt based stuff,and seems to work about as good if applied to a clean rust free surface..asphalt based stuff sticks to just about everything no matter what,including YOU!..
