I think all (including myself) have assumed you were airbrushing.
"Hot" thinners like GP are not really suitable for brushing, they evaporate too fast. This thinner attacks the existing coat and the physical dragging action of the brush will tend to dislodge some of the underlying coat. Giving you streaks.
For brushing you want something far less aggressive. Personally I use good old Mineral Turps for the small amount of brush painting that I do. $2 a Litre from Safeway. FWIW this is also what I clean my brushes with.
If your Aeromaster is an enamel then it should be quite OK.
