What do you guys do to treat rust underneath a car.
I bought a 2001 Hyundai Elantra 134,600 miles and want to work it up before winter.
Thanks

 

Are you talking surface rust? or severely rotted sheet metal? If the later, the only way to deal with the problem is to cut the piece out, replace, prime and paint. I would question a large investment for 8 year old Elantra.

Surface rust? More than 10 years ago I lived in more humid environment and dealt with surface rust using a product from the Eastwood Company that I think was called "Carrolless" or "Caroless" (something like that anyway) rust primer. It was a very thin (and messy) product I simply brushed on, let dry and then sprayed a grey or black enamel on top of that. The underside of the car looked like a pinto pony before I was done, but the treatment did a good job of stopping or slowing down rust spots under the car. A quick search of the current Eastwood site shows a rust converter primer available:

http://www.eastwoodco.com/jump.jsp?i...amp;itemID=372

I see this product comes in gallons or aerosol cans. I've never used this particular product, so I don't know how well it works. Eastwood does have a good reputation for selling products that perform as promised.

 

To just slow down surface rust under a car, run it thru the local car was & get the 'bottom wash' if they have it or at the local .25 car wash do your best to clean up the underside and wheelwells.

Get it home & leave it dry for several hours, and then have at it with a couple cans of Rust-O-Lium matte-black. Use masking tape along the rocker panels/body painted areas, and watch out for the over-spray. Spray the stuff on in heavy coats as you want it to 'soak' into whatever you spray it on.

 

I put some chassis saver on.
It was $40 per quart and used about 1/2 on this car
http://www.magnetpaints.com/underbody.asp

Rustoleum makes
http://www.rustoleum.com/CBGProduct.asp?pid=155
Needs a second coat on time. I would use bright red or yellow to keep track of what is painted.