Is this white latex paint or white oil based paint?
Because of the way latex paints form films, if you washed latex paint out of the brushes, then the white water that may have dripped out of the brush afterwards is NOT white paint because it won't have any coalescing solvent in it. So, the white drops will dry to form a white powder that you should be able to remove with a scrub brush.
Ditto for oil based paint. If you washed the brushes out with mineral spirits, and dumped the mineral spirits down your local sewer, then there won't be enough alkyd resin in the white drops to form a continuous film. What you're likely going to get there, again, will be a white powder that you should be able to scrub off with a brush. Your neighbors are going to think you're a jerk for pouring that dirty paint thinner down the storm drain tho.
If you have trouble getting either one off with a brush, then heat them with a propane torch to soften the "paint" and scrub it off with a steel wire brush.
PS: Next time save the mineral spirits you use to clean up oil based paint. In a few days the pigments will settle to the bottom of the mineral spirits, and you'll be able to pour the clear mineral spirits off. Actually, that liquid will be yellowish and clear. It'll be a solution of alkyd binder in mineral spirits, but it can still be used for cleaning paint brushes and other dirty tasks, like cleaning up after changing the oil in your car, etc.
Last edited by Nestor_Kelebay; 07-05-2010 at 07:41 PM.