I've painted with Kirker base/clear and have had excellent results with it. Durability has not been an issue. Cost: HALF of PPG. Kirker has price comparisons on their site:
www.kirkerautomotive.com. They only have a limited choice of colors, which is how they keep costs down.
A friend of mine painted an RV-3 with Kirker base/clear...showplane finish.
I turned another friend of mine who used to own a body shop onto Kirker, and he sprayed six gallons of it to repaint his motor home, and is very pleased with it.
Another friend of mine is now painting his Thatcher CX2 with Kirker, also is having great results.
You can get it online from smartshoppersinc.com.