The best stuff for what?

Fluid film is a great product with lots of good reviews.

Undercarriage coating and rust coverup

I bought a new gas tank and sprayed the whole thing (bare metal) in duplicolor undercoat. However, I can scratch it off with a fingernail very easily. I didn't think that I needed to prime beforehand but now I am wondering.

a primer prolly would have helped. im no paint expert tho. how long did you wait for it to dry before you touched it?

Hour or two.

it will toughen up a bit. mine took a while to cure good