Reply to "Undercoating R&R"

Nice first post Dan!

Sorry I did not get around to writing the follow up to my cavity wax post.

It sounds like you have done an excellent job. It's interesting to hear that your car had little or no rust under the original undercoating. Some cars are like that. If that is the case, then as George says, you might as well leave it alone. dont fix what is not broken. I think a lot of it depends on how long the bodys sat arround in bare metal down at that Vignale assembly plant in Turin.
My car had surface rust under the original undercoating on probably 80% of the car. So I had no choice but to strip it back to bare metal.

Here in the land of the black sun, they salt the roads throughout the winter. So when rust does take hold, it spreads under the undercoating. this is also true of powder coated tube chassis cars. It just creeps underneath.

Like Coz, I use a heat gun, but with great caution on the underside of the fenders. It would be possible to distort the sheet metal if it was held the same place for to long. And of course, if you are not repainting the car, it could burn the paint. I also use a Snap-on air powered gasket scraper. This is a great tool for undercoating. The thicker it is, the easier it comes off. But, as you said Dan, they can dig into the sheetmetal, so it is a bit risky on the under the fenders if you are not intending to repaint the car.
Sand blasters do not work on rubberised paint, the sand just bounces off.

Once the underside is back to bare metal I do my best to remove the surface rust. Sand blasting would be ideal, but I would never sand bast a car with the engine and tranny in place. Even then, it gets in everywhere and can be difficult to remove.

Electo chemical stripping, where they submerse the entire car is something I have no experience in. I have heard stories about people who had it done 5 or 10 years ago and are now finding the acid was not entirely washed out of there car, and is now actually causing corrosion deep inside the chassis sections. If anyone has any experience in this, I would be interested to hear from you.

I too like the 3M scotchbrite discs. But my favourite tool is the Black & Decker powerfile. I don't know if this was ever sold in the USA, but what a fantastic tool. I have been through a few of these now, and always cut the handle off it, and hold it by it's motor, this makes it a lot more compact. I also break one of the ribs off the back of the motor so that I can get oil on the rear bearing, this makes them last about 5 times longer. It uses a 1/2(13mm) wide belt. When I weld a piece of metal together, I grind the weld down with a 3" cut-off disc in an electric die grinder and then finish it of with the power file.
This type of tool is also available as an air tool. But the electric one is much lighter, and it keeps your hand warm, instead of freezing it off!

Once the car is down to bare metal, I thoroughly wash it down with thinners. I try not to leave bare metal any longer than I have to. If I ever do sand blasting, I always prime it the same day to prevent flash rusting.

As for primer, I have tried lots of different types. I used to cut out little squares of metal, paint each one with a different type of rust preventing primer or liquid, then give them a top coat and hang them out side for a few years.

I came to the conclusion that the metal preparation is more important than the type of "rust preventing" paint you use.
However, my favourite is self etching primer. The same type that the body shops use when they do a bare metal repaint. This is a 2 part product, Paint and catalyst.
This is much better to spray on than to brush on, because it is quite thin. But it is possible to apply with a brush. I have not tried SEM Shield. Is that a primer type paint or gloss type paint?

I would then spray over that with 2 pack (polyurethane) automotive paint, The same as you would use to paint the rest of the car. If you wanted to use cellulose (lacquer) that would be fine too. but do not use synthetic (enamel) this stuff is cheap for a reason. What ever you put over it,(like undercoating) never really bonds to it.


Now for the undercoating, It's a good idea to scuff the surface of the paint to covered with a scotchbrite pad, that way you get a chemical and mechanical bond.

I also like the 3M products. but have found many others to work great too. If you want to paint over the undercoating with body colour paint in areas like the chassis legs around the back of the tranny, use the Wurth stuff from Germany. They manufacture one that can be painted over, whereas most others crinkle up.

I apply the undercoating with a schutz gun and find it most satisfying!


Johnny