gas cap leaks

[Buckman]

I've replace my gas cap twice because they both leak down the side of my 71 gmc. I cut a gasket and but it in the cap and it stop for a while but started back later. I would like to get it fix before I paint the truck, what should I try next?

[pjmoreland]

The gas cap on my truck leaks too if I tighten it on all the way. If I tighten it on about half way though, it doesn't leak. It gets tight half way on, and then starts getting loose again as I turn it the rest of the way on.