I have the same problem trying to hand brush acrylics. That's the one thing I hate about using acrylics, it is real tough to get a good finish when hand brushing them.
The retarder and primer are both great ideas and will help a lot. The retarder slows down the drying time and the primer gives the acrylic something to hang onto. The best advice I can give is to thin the paint some so it will flow pretty well, and get it done on the first pass. Everything is fine as long as you don't make subsequent brush passes over the first pass. Get it covered on the first pass and move on and everything should be OK.
Also, watch for "Skinning" on the bottle or palette or whatever you are using to get paint out of. Acrylics dry so fast I usually get one or two brush strokes and when I go back for more paint the surface has already skinned over and dried a little. If I have to hand brush acrylic I usually pour a little out into an old paint cap or something, add a couple of drops of water and paint quick.