Liquid Tape can be purchased at most retailers and hardware stores. Mine is "Performix" brand that I picked up at Wal-Mart. They run a little over 4 bucks a bottle and underneath the cap is the paintbrush applicator. It goes on pretty well. All you have to do is remove your O Ring and apply it evenly around the end of the plunger tube. Get the first coat as even as you can (if you can still see orange, it's too thin). It'll take forever to dry if you use too much though, so don't put this on in gobs. After 4 minutes, apply a second coat to finish it up. The bottle says to let it dry for 4 hours for a permanent set. After that, put your O Ring back on and add a little bit of silicone grease. This should be a near-perfect seal for Reverse Plungered blasters. It may require different thicknesses on other types of blasters.
Liquid Electrical Tape can be removed relatively easily, so if you find you've applied too much, you can get it off after it has dried. Don't try to take it off while it's still wet. It makes an awful mess. The one you see in the pictures was for my Longstrike Blur. While the Stock Spring that I tried it out on was too weak for this perfect seal, the Orange Mod Works Stage 1 Kit for the Longstrike pushed through it just fine (and is quite powerful too!) This version had 3 coats on it. I'll be experimenting more with this Liquid Tape and if I find any other good uses for blaster modifications, I'll post 'em up here.