You don't need to thin it. If you want it thinner heat it. I heat the wood first then apply the glass, then heat that. You can get it to flow like water into the wood. I use heat to thin it to get a little better penetration into the wood and into cracks. When trying to strengthen a stock head I have even gone as far as curing it under pressure in a big pressure pot I had. Tried to drive it into the wood more. Never failed so I don't know if that helped at all but seemed like a good idea at the time.
The only failures I've had have all been due to inadequate preparation of the wood or joint and not getting great clamping action. Oil, grease and dirt are your worst enemies when gluing any wood. Then comes moisture if you are not using a ureathane type glue which loves a little moisture. If you are working on your own stock take as much time as you need to make everything as perfect as you can. No corner cut is going to make the job better.
