what is the best way to fix the rust on the bottom of my door. should i sand blast it then bondo it. i am going to have bedliner spray all a long the bottom of the whole truck.

 

Nothing will stop rust except removing the rusted area and replacing with new steel. Rust is like cancer, and will fester and spread. Bondo will make rust worse by trapping moisture between the bondo and the rust. Bondo was not designed for covering up dents, body damage, or rust, it was designed to fill small imperfections in the body prior to paint. Sandblasting will only clean the surface of the rust, but will do nothing to slow it down or stop it. Spray liner over rust is probably the worst thing to do, as it will amplify what the bondo will do over rust by keeping moisture in.

You need to cut away the bad spot, replace with new steel, prep, primer, paint, then you can put on your liner. Or buy another door. Do it right the first time, or it will come back to bite you in the ***.

 

What about a product like Por-15 or Rustbullet or similar?

 

POR-15 is good stuff, but still, it doesn't stop the rust, it only slows it down by sealing it from moisture. But metal sweats like anything else...so it will continue to rust beneath the sealer. I think POR-15 has a better purpose of sealing in clean steel, rather than covering up rust. I don't really use it, but a friend who has a hot rod shop loves it for fresh steel. I stopped in a few weeks ago and he brushed POR-15 Gloss Black on a flat firewall of a 1930 DeSoto Coupe...after it cured, it looked like it was sprayed, wet sanded, & buffed. No runs, no orange peel. Was very impressive. It should keep that piece of metal clean & rust free for many years...or atleast much longer than conventional auto paint as a coating/surface protectant. I do custom sheetmetal repair & metalshaping on old hot rods and antique/classic cars. I've not found any product to stop rust other than replacing the part or patching in new steel. And even patching new steel is only beneficial when done the RIGHT way, by butt welding and metalfinishing, not overlapping, tack welding, gluing, or smearing body filler over the joint. Seems to be a lost process these days by people who don't have the skill to do the job, or don't have the common sense to understand what they are doing is going to create bigger problems in the future. I get customers who ask "how cheap can you repair my rust for?", my reply is "it wont be cheap if you want it done the RIGHT way".

Sorry for the rant...I'm just bitter about bondo, haha.