Once the gun's innards are clean, take a solvent dampened rag and wipe the outside surfaces of the gun and cup. You could also take a bristle brush wetted with solvent and scrub the outside of that baby clean as a whistle. If you're like me, and just want to take care of the really important stuff, you'll hurry on to the next step, which is cleaning the air cap.
To clean the air cap, you've got to remove it from the cup and immerse it in clean solvent. Here, a bit of scrubbing with a small bristle brush is recommended, since the paint residue on the air cap is usually dried on. If any of the small holes (fan, stabilizer, annular, and primary jets) are clogged with dry material, you'll want to let the cap soak in the solvent until the material softens and flows out of the holes. If you find it necessary to ream out the holes for any reason, always use something soft like a toothpick or a broom straw. Never use a piece of steel wire (or, God forbid, the tip cleaner for your torches), as it could damage the jet holes by enlarging them, resulting in a really messed up spray pattern!
Once the gun and cup are cleaned up and looking all spiffy, the next step is to re-lubricate the gun (that's right, I said lubricate). Cleaning the gun with solvent will remove any and all lubrication a gun may have, and believe it or not, spray guns need lubrication, too (I may not keep the exteriors of my guns totally spotless, but I do make sure they operate to the best of their ability).
There are four lubrication points on the average spray gun. The first three can be lubed with a non-silicone/non-petroleum spray gun lube (available at any auto body supply jobber). The three items to be treated with this type of lubricant are the air valve packing, the fluid needle packing, and the trigger bearing screw. The fourth item is the fluid needle spring (located inside the gun body, behind the fluid adjustment knob), and this should be lubed with a small amount of petroleum jelly or non-silicone grease, like Lithium. If you follow these gun maintenance tips, you'll be sure to have a spray gun that'll seldom let you down.
Problems, Causes, And Corrections
While we're talking about spray gun maintenance, this would probably be a good time to touch base on a few common spray gun problems you might run into, as well as their possible causes and corrections.
Fluid leaking from the fluid needle packing nut could be caused by either a loose packing nut, or packing that's worn or dried out. This can oftentimes be corrected by either tightening the nut or by lubricating or replacing the packing.
Air constantly leaking from the front of the gun could be caused by a number of things. It may be due to a sticking air valve stem, which needs lubrication, or crap in the air valve or on the valve seat that needs to be cleaned out. A worn or damaged valve may need replacing, or a broken air valvespring or gasket that may have to be replaced could be the culprit as well.
A jerky, fluttering spray could be caused by material that's too thick. It may also be caused by a clogged cup cover vent, the pickup tube resting on the floor of the cup, or a damaged gasket between the fluid tip and the gun body.
If your spray gun exhibits a spray pattern that becomes much heavier at the top, bottom, or either side, you'll want to check to see if the air cap or fluid nozzle is causing the problem. You can do this by spraying a test pattern on a piece of scrap material. Then, rotate the air cap one half of a turn in either direction. If the defect in the pattern is inverted, then the obstruction is in the air cap and it needs a good cleaning.