Try to find 'Power Putty'
http://www.powerputty.com/ it is sold to rock climbers for grip strength and to guitar players for finger stamina.
You can also get it at medical supply stores for Physical Therapy.
Comes in a square box with a silver clamshell fist inside.
You get 4-5 eggs worth of silly putty inside for about $8.
The dr green is a little too stiff for masking purposes, but the green and dk blue work well. I have not ever used anything lighter.
I only spray acrylics, so I can't speak to how well it reacts to enamel paint or thinner.