Ways of dealing with rust on sheet metal
I have a question about a friend’s 1959 Thunderbird. He handed it over to a shop that very eagerly got started stripping it and immediately used a DA sander on the rear quarters. Then work slowed and two people quit. It has now been stripped for at least six months and he is concerned about rust when they finally prime the car. Thankfully, it is indoors in dry Southern California, and as of a couple of months ago it was still visibly clean. I know the best practice is to paint the car immediately after stripping, but when things are out of your control like this, can something simple be done to ensure a rust-free surface for priming? I understand that muriatic acid is used for acid dipping. Could the panels be wiped down and washed off before priming?
The simple answer is no. There is no magic solution that will wipe off the “surface” rust that would have built up on sheet metal left bare for the amount of time you state…not if you want the paint job to last. Rust develops in pits in the metal, and though it is not apparent to the naked eye, it is there to work its evil. And muriatic acid is not recommended for removing rust from automotive sheet metal. It is too dangerous and corrosive for such an application.
There are good brand name rust converters such as POR-15, but they only stabilize rust; they don’t eliminate it. If you are going to all the trouble and expense of doing a proper bare metal re-spray, you will want to apply the paint only to clean white metal with no trace of rust, because rust is relentless. Unless it is eliminated it will spread and ruin the paintwork that covers it, because moisture will get in through tiny cracks or pores in the paint. Also, in your locality the color coat has to be water borne to meet environmental regulations, so if the metal isn’t perfectly clean and primed and sealed you will have problems.
Tom Horvath, with whom I wrote the book “Pro Paint & Body,” and who is one of the top auto painters in California, recommends that you finish stripping the car with aircraft stripper and then have it blasted with walnut shells or some other dry, mild medium to eliminate any rust before you apply a waterproof epoxy primer. Sand blasting will thin, harden, warp and peen sheet metal, so never use sand or other harsh media for this job.
It is also possible to effectively eliminate the flashover rust by using a sander or block sanding by hand with 300-grit open coat sandpaper, but that would be a momentous amount of work. I know this because I have done it, and unless you have help, you will have to start over by the time you are finished. If you want to know the stripping and painting process in more detail get our book. Make sure you get the second printing with the blue cover which includes information on waterborne paint as well. It is available from Amazon for $19.95.