Ok, a year ago I heard about a spray can solution that could stop rust from spreading or getting worse. I dont remember the name and I was wondering if anyone else knows about this stuff or their web site.
Eastwood's Rust Encapsulator can be bought in cans. Just brush off the loose rust and spray. It works extremely well; I did my floorpans 7 or 8 yrs. ago and still no problems.
The stuff that I am talking about is able to be sprayed ON rust and keep it from spreading or getting bigger. It was a black paint and it wouldnt chip off or crack if the metal was bent or hit with a screw driver.
The product I spoke does prevent spreading and it can be had in black or a reddish color that looks like the original underside of our cars. You do spray it on rust but it is a good idea to brush off any loose rust so it can get at the real stuff; only takes a minute.
B.T.W., I have never noticed any spots being "scratched" or coming off under the car after normal driving but I guess if something really big hit it, it could happen. If that is a major concern, you could probably apply rubberized undercoating out of a can to add some "flex". Just read the instructions on the can for compatible topcoats.
Just as a heads up that stuff form wal-mart smells like rotten cat “PEE” not something I would wont to work with in a confined space. The eastwood stuff is pretty good I have used it and seams to work well and just an FYI you can weld through it if you must. Just my .02 cents