Ron,
For primer, try a 'metal etching' type found in quality auto stores. Most come in a spray can. May be able to find it in 'normal' smaller quantities that require thinner. If able, spray through your airbrush as usual. The spray can type should be thin enough to airbrush...I used to just spray into the color cup and shoot away, directly from the spray can.
The metal etch primer will give an excellent 'tooth' for the paint to adhere. About any kind of paint would seem to do for color.
Also, I will bake the paint when I get to that stage with my project. It worked great on old John Deere farm toys I used to restore.
I've used the Testor's clear and flat on projects-acrylic and laq type. Great results with both.
I use/prefer "Future Floor Wax" for a smooth gloss finish-decals love it. With a little setting solution, you'll never see any film after dull clear is applied. Spray it 'full strength' for the bottle.
As for Scale Coat, I remember reading in one of the Weaver threads someone mentioned all the paint was sold out. Some dealers may have some left, but Weaver itself was said to be out of stock.