Undercoating help?

I am looking to do some preventative maintnance on the under side of the xj. It seems that right where the muffler is and exactly across the otherside, rust is appearing at the seam of the floor. Nothing bad or any holes, but i wanted to get something on it asap.
Do you think some roofing compound would work? something that would dry and harden? Or does that undercoating spray really work?
Ideas?

Roofing compound or plastic roofing cement will melt and make a royal mess.

I've used the napa brand undercoating with no problems yet.