i have alot of experience with fighting rust when restoring my old vw's ,
there is no such thing as a chemical rust killer, it merely encapsulates it and converts it, anything short of getting a wire wheel out and doing it right may still have issues in the future and require upkeep, depends on your needs how much rust and all of that, here is a shoot out a guy did of various rust products
http://www.thesamba.com/vw/forum/viewtopic.php?t=131753
other products people use
http://www.halonmarketing.com/mall/Rust Away & Metal Prep.php
http://www.eastwood.com/eastwood-rust-converter.html
http://www.rustcheck.com/howworksasp.asp
personally i'd just clean it up right with a wire brush or softer grinder and then spray with some por-15 then rhino line to go the extra distance, you should post some pics so we can see how bad this is.