Marc...
If you really want to look rather innocent in traffic, but want a true classic, consider Sepia Brown!!!!

Nice project. I sent on an e-mail over some spare parts.

Bob
73.5T

There are better paint guys here, real artists, but I'll take a crack at this.'

If you are doing a total repaint, you strip the car of everything that is screwed bolted or glued on. So, that's a bit of work. On a 911, you should also loosen the fender and pull the 'welting' between the fender and the cowl. Applying clear is just like applying a color coat in terms of coverage.

Why do you want more paint?? The manufacurers have specific standards for finished paint thickness, in mils.

Is your current paint in good condition? If so, stop in to your local body supplier and ask which clear they recommend. Then follow their printed instructions.

It is a great chance to block the color.

I've heard that there is a time frame in which you have to apply the clear coat over the base coat to get the proper adhesion but I don't know the story for putting clear on a clear that has already hardened. I would imagine that you would have to either roughen up the clear with either sand paper or some kind of etcher. I think you would also need to stay with the same product as different product brands aren't always compatible.
Hopefully one of the paint gurus will chime in.

Black is always elegant.