well, them there "art store" brushes............aint good fer 1 shot.........ya gota have them "special sign painter brushes" that hold a gallon a paint each time ya fill em up!!!!!! hehehehehehehehheehehe.
SORRY devil made me do that)))))
sound to me like the brush you have is not the brush you need. are you using a TRUCK FLAT, or a QUILL???
as for thinning one shot, DO NOT OVER THIN!!! then the paint wont last.
1. get a plastic 3 section picnic plate.
2. in one of the small sections put the 1 shot...
3. in other small section put the (turps, minerial spirits, gas, automotive enamel paint reducer), you only need a TEASPOON at most.
4. get a clean #8 gray quill, just touch it to the reducer, then dip it in the paint.
5. in the big section take quill and flatten it and pull a little, then flip it over and do the same on the other side,
6.keep adding paint to it.....and touching the reducer, til it feels like its soft butter on fresh bread.
NOW YOU GOT IT...........take it to object to paint