There are spray paints which bond directly to plastic. You could also try plastic sheet in the future for a nice finish.
Those paints (like Krylon Fusion) use welding solvents to slightly melt the paint into thermoplastics (like ABS) to form a direct bond. But epoxy putties are thermoset
plastics, which isn't capable of being welded or remelted. The special paints can't bond to them any better than regular paints.
If you want to improve the finish of epoxy putty, then you've got to choose whether to add a coating (like paint or lacquer) or to mechanically polish the surface to a better finish.