Harold, interesting approach to get that camou applied.
Since it is hard edge, you could just hand paint with a brush, though it would be advisable to thin the paint such that you will apply at least two layers. This avoids the chance of paint buildup over details, and results in a nice even layer.
But I can see the advantage of preparing masks. It allows one to concentrate on the paint application and not worry about camou shapes.
regards,
Jack