The painters were ORDERED to twirl the tips of their paint brushes that way.
When I ran a pro photo lab, one of our products was Cibachrome, prints made directly from transparencies. Dust marks on the transparencies showed up as black marks on the prints. The people doing the dust spot correction used an acid to bleach out the black mark to white, then used color dyes as necessary to fill the white dots in.
It was hard to break the habit of twirling the brush tips on the tongue, but sooner or later the acid made its point.