Is it really necessary to do bare metal prep?

[Biff]

I'm going to take on a repaint of my old '70 911 this fall. I've seen alot of questions about paint stripping of car to bare metal using aircraft stripper. Is it really necessary? My father was a coatings engineer his entire career, when I was a kid we painted alot of our family cars. He had told me years ago that it would actually do more harm than good to take it to bare metal because the factory paint line had such controled conditions ie. humidity, temperature, moisture content of metal, paint formulations - hardeners, ect. He said that the best base primer is the original that was applied when the car was new. Any thoughts or was he way off?

[epbrown]

Biff Sr was right, but you have to take into account the condition of the paint. Paint that is solid but faded and dull makes a good base, but if the old paint surface has been breached (it's peeling, there is rust underneath, and so on) then anything short of going to bare metal is going to be a waste of time as the work will be undone eventually.

Emanuel

[Bob's Flat-Six]

This is from a 1980 Porsche Color chart.

[911pcars]

Painting over previous paint/primer that is in good shape is okay as long as the total amount doesn't get too thick. I forget the exact number (15 mils?).

Too much paint causes a tendency for the paint to crack.

Sherwood

[toolman]

Too much is 12 mils. It is OK to sand the bad areas down to metal and feather the sanded area out so no abrupt lines are created. Use a good epoxy or etch prime before you use a urethane filling primer.