I emailed my mentors (Christopher M. & Tom G.) the following question:
Do you think "undercover" (the rubber sealant sprayed beneath cars) would work well to seal the rust in the cavity beneath the gas tank / floor board -- basically the tubing that lies beneath the floor mat?
Or maybe a zinc product, which attacks and seals the rust?
I'd rather not pay the price of a can of Zero Rust in shipping alone ($8 for a can and $8 for shipping) and I am looking at the alternatives the auto stores have. Much of the area did receive over spray during paint and that went over the rust. PPG cleaner and conditioner reached the rust prior to paint as well.
This is an extra preventative measure I'd like to take. The bike will be stored in doors.
Here are the two separate answers I received (both saying the same thing!)
I think you should leave the tunnel alone. Do not
"seal" the rust in. Moisture ALWAYS penetrates
sealants. If you have paint on one side of metal, and
sealant on the other, the moisture has nowhere to go
to get out, and will accelerate corrosion from the
inside out. Leave the tunnel side exposed to air, so
any moisture that gets in there can evaporate.
The undercoating product would likely get onto the painted exterior
surfaces, creating disaster. It's horrible gooey stuff. Like you say, some
protective coating already hit that semi-rusty steel, so it's probably OK.
Plus, the fact that you'll store it indoors is good news. I wouldn't worry
about it. Maybe if you like you could get a can of fogging oil at a
snowmobile dealer or dirt bike shop. It's a fine oil that comes in a spray
can and you use it to spray in the intake manifold when a 2 stroke engine is
being prepared for long-term storage -- it coats the cylinder/piston with a
fine oil to prevent corrosion. Good stuff. Another alternative would be the
nasty waxy stuff you can get at auto paint stores (like my beloved BAPS)
designed to be sprayed inside auto sheet metal panels to prevent rust --
exactly the situation you have. But again, this will be an indoor bike, so
even if you do nothing you should have nothing to worry about.
Always double check with the painter and make sure to have the problem correctly resolved upfront or at least make sure over-spray coats the hidden areas. For this bike, we're leaving it as-is per the recommendations I received. Your situation may be different; therefore read Christopher's response closely for alternative solutions.