There are two problems you can have with rustproofing. not getting it done; and getting it done at the wrong place. Road salt and corrosion take a toll on hidden areas such as brake systems and gas tanks. Mechanics regularly see vehicles coming into their shops requiring hundreds of dollars' worth of repairs that could have been avoidable with proper rustproofing.
Many rustproofing professionals advise you to skip the dealer's rustproofing offer when you buy a car , since a dealer may charge you more than a reputable after-market company., and they don't do the work properly. Professional rustproofers use a special tool to cut holes in the frame and body cavities, and then spray rust-fighting compound through those holes. Less thorough shops often cut the holes using a standard drill, which leaves ragged edges where corrosion could start. You should also check that the rustproofer gets into troublesome hidden areas such as the top of the gas tank, which some rustproofers will often skip.
Check for endorsements (or lack of complaints) from the Automobile Protection Association, Consumers Reports, the Auto clubs, and the Better Business Bureau. You should also insist on watching the job and making sure it's done right. If a rustproofer won't let you watch as he operates on your car, you should probably find another rustproofer.