I use only Kadee semi-scale couplers, 153s prefered, 156s if necessary. I brush paint them with solvent based paints, usually a bulk pack at a time. I have found that using a dry brush technique allows for painting the entire coupler without gumming them up. Once they have dried for a few days, I check them for proper operation. Any that are stiff are easily corrected by grasping them by the shank and moving the “air hose” in a wobbling circular or stirring motion which restores them to normal operation. I have no coupling issues caused by paint.