If paint is leaking from in front of the trigger, in the packing nut area, first try tightening the packing nut. If you have to tighten the packing nut so tight that the needle no longer moves smoothly when the trigger is pulled, you must replace the packing.
If paint is leaking from between the canister and canister lid, verify that there is only one canister seal installed in the lid, that it is properly seated, and that it is not worn or damaged. If the canister seal is worn or damaged, replace it. (We provide two canister seals with every gun, but only one should be used at a time; the other seal is intended as a replacement item.) The canister seal is a white gasket, which should be placed in the groove of the canister lid. The clear plastic check valve is installed over the canister seal.
Check for dried paint in the area where the canister seal and check valve sit. Clean off this area and the top edge of the canister so that the two parts can achieve a tight seal.