I can't afford masks and suchlike and freehand painting CAN give a reasonable finish, but I use a method that works for me. I must point out that it will only work on double road wheels.
What I do is glue the two wheels together (you can strengthen them with superglue if you wish). Then paint a thinned (app. 20%) strip of paint with a wide brush on an old mouse mat. Stand the wheel upright next to the paint strip. Get a long ruler or other staight edge and slot one end of it in between the wheels and then hold both ends of the ruler and 'roll' the wheel along the paint strip. By applying a little pressure you can easily adjust the profile depth of the tyre. Try it. If it works tell your friends.

Tom