Abstract: A spill preventing diaphragm attachment for a paint spray gun paint cup as shown. The diaphragm is engaged around the mouth of the paint cup and forms, with the undersurface of the paint cup lid, a chamber above the paint in the cup that is deeper at one side than the other. At the deep side, oriented 180.degree. away from the vent opening in the lid is a vent opening in the diaphragm. Any paint that might flow through the diaphragm vent opening as a result of tipping the spray gun drains back into the cup as soon as the gun is righted. The diaphragm is readily replaceable without tools.