Castings having the first light coating of grey undercoat applied, I find it better to give a couple of light coatings as opposed to one thick one. As KV pointed out, it’s best to warm up the can in the sun for 15 minutes or so to thin the paint, also it requires a good shaking before and during the spraying process. I use a car primer, which is ideal for metal castings, gives a nice even coverage and dries to the touch in 20 minutes or so. The castings are turned over a number of times so they can be sprayed from different directions to achieve a total coverage.
Jeff