A liquid container system for a spray gun. The container system includes an outer cup, a collapsible liner for holding a liquid to be sprayed, and a lid for closing the liner. A mounting ring may be used in the cup to make locking engagement with the lid. A lip at the open end of the liner is supported by the mounting ring. A removable lid with a liquid outlet is inserted into the mounting ring, clamping the liner lip to the mounting ring and providing for leak-free operation of the system. Alternatively, the lid may include a cylindrical portion for making sealing engagement with the open end of the liner and projection threads for locking the cup. An adapter connects the liquid outlet of the lid to the spray gun. A collar may facilitate locking engagement of the adapter with the lid.