Living a few hundred meters from the Gulf of Mexico, you learn how to deal with rust quickly. The following is jsut my $0.02, take it for what it's worth.
There's a few ways you can go about it (these are just my opnions):
To neutralize your rust:
Good: Go to your nearest ACE Hardward and buy a gallon of Oshpo and a few plastic spray bottles.
Better: Go to your nearest Home Depot and buy a gallon of Rust Bullet and all associated application products:
Rust Inhibitor Paint | Stop Rust | Rust Corrosion Inhibitor | Rust Converter
Best: Go to the link and buy the rust Prevention.inhibitor from Eastwood.
Rust Repair - Auto Body Rust Treatment - Corrosion Treatment - Eastwood
Take a STOUT wire brush and knock off all the rust you can get to. Doesnt have to be perfect, jsut get the flakes off. Pressure wash the underside of your rig, mask the driveshaft, U Joints, exhaust, fuel lines,sensors, etc and start spraying everything metal. You can also use a cheap paint brush to spread it around for better coverage.
Then, you can apply a top coat. POR 15 works great,but you'll likely want to paint over the POR 15. But it's kinda like the Ford vs Chevy debate. All about preference.
I've had the best results (and longer lasting, better looking results) with KBS Coatings - Paint Over Rust - Stop Rust Paint - Gas Tank Sealer - Gas Tank Liner
They also have a great line of rust inhibitors.
You can also look for Marine Grade Rust Inhibitors (I used that on my gennie '32 Ford Coupe and was impressed with the results, but it was EXPENSIVE, and I have to sand it all off again, because it went on slightly blotchy (not condusive to good paint job).
again, jsut my $0.02!
There's always Kersosene!