Actually, this is too early to say how well it works, but I did buy a
quart of POR-15. (POR stands for "Paint Over Rust.") Never found the time
to finish the project, but I did paint the old propane tank and about 20%
of the underbelly with that stuff. (only used up only about 10% of the
can.) The time consuming part of the project is to remove all the old
undercoating, although there is no need to sand down to bare metal. As the
name says, you just paint over the rust. 

POR-15 dries to very hard, ceramic-like finish. It ain't pretty, but since
it's for the underside, I don't care. I have not applied any rubberized or
tar undercoating; this way I can keep better watch on rust. So far, POR has
held up very well. I'll report again after next winter (hopefully the last
one in the Midwest :-), but I have a good feeling this stuff is made to
last. 

Sami ('75 Westf.)
dakhlia@wuecona.wustl.edu