Primer weight

Anyone have an estimate of how much weight can be saved by only priming interior mating surfaces vs all interior surfaces. (Sherwin Williams wash primer)

S/W wash primer adds very little weight if applied correctly. You need very little product. Spray a light coat and compare it to bare aluminum. If you can see any difference, you have enough.

P60G2

Mel is right on. It's so light, it's probably not worth trying to keep it just on fay surfaces.
I calculated it once. Huge swag. 0.3 grams per sq/ft of area. I also calculated the area on my 7A for paint estimates. I came up with about 4 ounces weight for P60G2. Epoxy Primer and urethane paint for the exterior would be significantly more.
The square footage is on my Basic Builder Log. Available for download on my blog. I am revising it for weight calculations.