Bumped into USC All-Metal and did some reading. Aluminum based and you can powder-coat over it. Not chrome though.
Used it to do some grill work on my '54. A few observations:

* The only negative is that the reactor is pretty atomic. they say put 12-14 drops into a golf ball size of the filler. Too much. It only gives you working time under 5 minutes. I went to 8-9 and it was better but you still need to work pretty quickly.

* Did a coat...sanded it...then did another coat to build it up. These were on dents NOT holes.

* Hardens in about 20-30 minutes to a point where you can sand it. I used a circular grinder with 80 grit at #4 (of 6) speeds and it cut thru the All-Metal and feathered it very nicely.

* I am thinking if I was to fill a fender hole, I would start on the bottom of the hole with fiberglas cloth and filler (like DuraGlass) to create a backing. then I would use All-Metal to fill the hole itself.

Anyway, good product that is a bit pricey but satisfied so far.