Paint dispenser thoughts

I was just thinking of ideas of how to dispense acrylic paint. My brother and I have been working on this on and off over the past while and have been thinking about this simple design which he came up with.

The lower tube will be a syringe like tube with a plunger.

If the vertical shaft is a 1/4-20 threaded shaft, and those two circles are gears perhaps we can drive the threaded shaft back and forth. With this simplified concept, if the large gear is fastened to the shaft, then we can turn the shaft but the shaft cannot idle back and forth since the gear is sandwiched between the two mounting brackets.

If we put the lead-screw in the middle of the large gear then it can idle the shaft back and forth. But then something needs to stop the shaft from turning.

I’m trying to see if I can get out of having the shaft being attached to a pair linear rails. Rails of this size are not expensive it’s just a lot of parts. If we go with 6mm rails we can buy bearings for 5 bucks and the rail is 24 bucks for 30″.