Hi Sam
I’m with Dave on this, pre cut masking is very useful with the saving in effort and time it gives you masking up for airbrushing. However, it is not that helpful if you are using a hairy brush. If you are brush painting you need several things to go right.....you need a decent brush that holds a point, paint at the right consistency so it covers ok but flows well, and a little technique when painting....the technique is to unload the brush before starting so the paint flows off the bristles when you want it to, not floods off as soon as you touch the surface. To do this, dip the brush in the paint, paint a couple of short lines on photocopy (or similar) paper to unload the excess paint, then, and only then, start painting the model. You may need to go over some areas more than once to get full coverage, but that is usual with most paint. The last tip is to have another fine brush to hand that is damp (not wet) with whatever thinner suits the paint. Use that to tidy the lines if they overshoot while they are still wet. This lot should make things a little easier....
Cheers
Tim