preventing frame rust

Hello all, I have a GMC that sits quite a bit and the body is very nice but I want to make sure the frame doesn't get real rotted. I've heard about places that spray oil but I was curious if any members had any suggestions other that just spray cans of undercoating. Thank you

I'd put a tarp under the truck full length to block moisture from leaching up under the body, too late for any undercoating.

This site has some good links referencing issues like that.

http://www.insider-review-forums.com