First of all, do you really need a servo ? I guess all you need is an on/off function, right ?
I see two options
1. Use a solenoid valve to control flow of air to the paintgun. Use a string or clamp (or any other method) to keep the valve on paintgun in open position.
2. Actuate the valve. Just as you wanted to do it in the first place but use a car lock actuator
They are really powerful. Simply apply current and it extents. Change polarity (or use other wire) and it retracts
I'll post a video of it later (if I manage to find it)
Here a guy uses it to open a valve on a spudgun.http://youtube.com/watch?v=JyMpMPD7Tv0