Snowy:
You are not alone in this boat. I have had this happen to me quite alot as well.3 things you can try. That worked for me
1: Try thinning the paint a little bit more and try to get a little closer to the area.
2: Try adding an Acrylic retarder to the thinned paint to extend the drying time. This can be found in any arts and crafts stores.
3: If the paint is already on the model like the one you mentioned. Gently rub the area that is effected with 000 or 0000 Steel Wool. Not alot of pressure is needed just the weight of the wool itself moving over the paint should smooth the paint out without removing any of it. Just remember to dust the model off before clearcoating it.
Good luck and hope this helps you out
Paul