RESTORATION PART 3
By Skip Cain
The most expensive and time consuming work on a restoration is rust repair and panel replacement. This is what every buyer of a classic car hopes to avoid. Not only is it expensive, it is also hard to find a shop that wants to deal with it. Most body shops thrive on the quick in and out collision repair. This is why you should always seek a shop that can handle proper rust repair and panel patching and replacement.Check out the shop, if you see a mig welder, well that's a good sign. Ask direct questions as to how they handle rust repair. If they tell you they fill it with filler or fiberglass, run away and don't look back.
There is only one way to find out how rusty your car is. Most of the time rust is hidden under filler and paint. I can't tell you how many times I've been told " there's no rust on the car." only to strip it down and find plenty. Hey, these are 25-30 year old cars, there will be some rust, that is expected. The idea is to find the rust, remove it, and try to prevent it from coming back under a new paint job. Let's get started.
For a complete body and paint restoration the whole car must be stripped down to bare metal. You want to get off all the old lacquor primer, bondo and paint. I'm lucky to have a chemical stripping company nearby, and I have the front cap stripped. For what it would cost in time and labor to strip, I put it in the tank.When it comes out, all the paint, bondo, fiberglass, seam sealer and rust are gone. The sheetmetal looks new and shiny except for where rust was.All rust and collision damage will be apparant now, and I've seen some humerous reactions from people who refuse to believe that the bottom of their fenders are gone.I don't strip the car bodies because it isn't cost efficient (unless it's extremely rusted out) Chemical stripping also removes seam sealer from inaccessable areas such as inside the quarter panels. If they don't have to be replaced, 30 year old sealer is better than no sealer.
Other methods of stripping are media blasting, brushing on Aircraft stripper, or the old tried and true sanding method. I prefer this method, and use a 8 inch dual action sander with 36 grit. Make sure to use a dual action (DA) sander, anything else( like a grinder) will leave gouges in the sheet metal. I go over the entire body with 80 grit on the DA after stripping with the 36 grit. This will smoothe out the surface and prepare it for the first coat of metal primer. To prevent flash rust, it is best to prime ASAP after stripping, as bare metal will flash rust overnight. Also, any panels that are chemically stripped in a tank should be scuffed so the primer will adhere properly. I recommend a self etching two part epoxy primer , such as PPG's DP40 on the bare metal. This will etch any microscopic rust, and really sticks to the metal. After the complete car is stripped down to the bare metal, and primed, you are ready to tackle the rust repair.
This is the point where the decision to replace or repair sheet metal panels should be made.
It would seem easier to replace a rusted or damaged fender with a new one.I should mention that my own preference is to try and use factory replacements, if possible. They fit better, and are of better quality then aftermarket parts. Aftermarket replacement panels are currently better than they used to be, but still require a lot of fitting to align properly. If you have a factory fender with only some minor rust damage, repair the damage. If you must replace a sheetmetal part, and a factory (NOS) piece is not available, or too expensive, then you'll have to deal with the aftermarket part. Most GM A bodies rusted in the same places, so there are a lot of replacement parts available out there. Year One has a sheetmetal rating system of 1 to 5, 1 being a NOS or factory panel that is perfect (I haven't seen one yet) Most of the aftermarket panels are rated as a 4. It is noted that panel distortion and waviness is to be expected with these panels.
This means that these panels will have to be skimmed with filler, so if you don't like bondo you won't want to use these parts.
Rust repair will involve cutting out the rusty metal and welding in new sheetmetal. Rust is the formation of oxides in metal caused by the presence of water and oxygen. These ferric oxides cause corrosion, first in the form of pitting, which then turns into holes. Salt will hasten this process, as anyone living in the "rustbelt" knows.
Caution must be used to make sure all the rust damaged metal is cut out, especially on panels that aren't removed from the car. A lot of rust can be hidden behind the panel. Consider it an operation to remove a cancer, best to cut out some clean metal then to leave some "cancer" where it can spread again. I use a cut off wheel or a pneumatic hacksaw to cut the sheetmetal.
Make sure you don't cut too deep,as you don't want to cut through any support brace. Make the cut as symmetrical as possible, as this makes it easier to measure and patch. At this point, after the rusty sheetmetal is cut out, and before the patch is welded in, take some rust preventative measures. If you can, take the same measures after the patch is welded in. Coat the back of the panel and the patch with a rust resistant coating such as PPG's DP 40, Eastwood's Corroless, or POR. A lot of shops don't do this, so you should insist they do. All is fair in the battle against rust. You want to use a butt weld for your patch. To reduce "high spots",excessive grinding and weakening of the welds, flange the edges of the patch and panel 45 degrees . This will create a V seam which is then filled with wire weld.
I usually tack the patch first to make sure of a good fit, and also for any last minute shaping. After the patch is tacked in, do a full weld around the whole patch. If it is on a large expanse of flat sheetmetal, use a lower heat setting or a heat sink, so as not to warp the surrounding panel.
After the weld is finished, grind the tops of the welds off so that it is just lower than flush with the rest of the panel. Use a pick hammer to tap down any remaining high spots. At this point, I will apply some self etching DP primer on the welds and patch. When properly dry, spread a waterproof filler, such as Duraglass by USC, over the welds. You should only have to use a skimcoat, no more than 1/8" thick. After this is sanded down, spread another skimcoat of premium plastic filler over the entire patch. This is then sanded and "blocked". Now it is ready for 2-3 coats of a high- build urethane primer such as PPG's K200, which is also sanded and blocked with 400 grit until all sanding scratches are gone.
When cutting a patch, make sure the sheetmetal is of the same thickness and carbon content as the panel to be repaired is. If it isn't, it won't weld properly. The patch may melt while the panel is just heating up, or vice versa. I have had this problem with some of the aftermarket patch panels that are available from the restoration companies. I usually cut patches from "donor" automotive panels, making sure they are rust free.
Another problem I have seen, involves the use of rust removers and metal prep. These chemicals contain phosphoric acid that dissolves rust. A lot of "do it yourselfers" are using this stuff to remove flash or surface rust from sheetmetal. The problem is they aren't removing the residue.
It goes against logic to wipe down a bare metal panel with water, but it is the only way to properly neutralize the acid. If left on, it will cause adhesion problems with the paint, or bubble up like rust. One guy came to us wondering why the rustoleum paint he primered his car in was lifting and bubbling. He meant well, but he didn't neutralize the metal etch and it reacted with the rustoleum paint. ( Use the rustoleum on your patio furniture, it's not compatible with automotive paints.)
My next article in this series will be about painting your restored Classic.