It's best to fill the plunger pump with sewing machine oil, or any lubricating oil that lubricates mechanical parts. Before each use and after cleaning the machine, add 1 ml (5 drops) of lubricating oil to the plunger pump's filler neck. The amount should be about 1/4 of a soda bottle cap. This will extend the plunger pump's lifespan. Do not add too much lubricating oil to avoid dripping into the paint can.
By vevor