One of the most common issues with airless spray pumps is where the airless paint sprayer will not prime. Priming an airless paint spray machine is the process where paint or solvent is drawn into the airless pump expelling all air until the paint or solvent has completely filled the pump. With most airless spray equipment this is achieved by way of a bypass valve and a bypass return hose so once the airless pump is completely primed a steady flow of paint or solvent is returning back to the original point such as bucket or paint hopper that the pump is drawing from.
When an airless paint spray pump has not been stored properly it can be very common for the machine to not prime. In almost every case this will be caused through the internal of the pump drying out and either the inlet valve or the outlet valve or both valves inside the airless pump sticking together, usually with a residue of paint. Even if an airless spray pump has been flushed out with water to the point where the water appears to be clear there may be still enough paint residue remaining to glue the airless pump valves together if allowed to contact air and dry out.
If the airless machine is not priming after storage you will need to locate the inlet and outlet valve and ensure they are not stuck together. Hopefully the airless pickup has not been placed in paint already, this is a good reason to always test an airless pump with solvent before placing the pickup in paint, as some disassembly will now be required. If you are not sure where these valves are located in your airless pump find the owner's manual which should have exploded parts few indicating where the inlet and outlet valve are located.
If you have a piston airless paint sprayer there will be usually always an inlet valve ball and seat located at the bottom of the pump and an outlet ball and seat located at the bottom of the piston. With most piston airless paint spray pumps there will be a connection connecting the pickup pipe or siphon hose to the bottom of the pump which needs to be removed. After removal the inlet seat, ball and usually ball page which stops the ball travelling further into the pump should be removed. Check that the inlet valve ball and seat are not stuck together, are not damaged and that there is nothing obstructing their proper operation. While the inlet valve assembly is removed from the airless pump the outlet ball operation can usually be checked by looking further up into the pump past where the inlet valve assembly has just been removed. Sometimes laying the airless pump on its back will assist in checking the airless outlet valve which is located up inside the end of the piston. Check that the outlet ball is free and not stuck by jiggling it with something such as a pen or pencil. If it is stuck something more robust may be required to free it such as a blunt pin punch. Once both inlet and outlet valve have been freed reassemble the airless pump ensuring that there are no damaged O rings or seals which could be causing air leaks and try priming your airless in solvent again.
If you have a diaphragm airless paint pump the process is similar but consult the parts diagram for your diaphragm airless pump to locate both the inlet valve and outlet valve. In these types of airless pumps the inlet valve is usually located under or behind the suction hose pipe and this can be checked by simply removing the suction hose or pipe and ensuring the valve is operating by pressing the internal assembly was something like your airless spray gun filter or the back of a pen. Sometimes if it is really stuck you may need something more robust to press on and free the inlet valve. Some diaphragm airless paint spray pumps these days actually have an external actuator that can be pressed to free stuck inlet valves. The outlet valve on a diaphragm airless paint pump is usually located nearby the inlet valve and usually requires removing the outlet valve assembly or the stop nut unless you have an airless diaphragm pump with an external actuator to free the stuck outlet valve ball.
Some people may suggest hitting your airless spray pump with a hammer if it won't prime. While this may work in some cases I don't think it's really good idea. I have seen airless pump cylinders cracked, distorted and irreparably damaged from being hit repeatedly with hammers. I wouldn't hit any other mechanical device or tool with a hammer if it didn't work and really don't recommend hitting your airless paint spray machine with a hammer if it doesn't work. Always test your airless paint spray pump in solvent before moving to paint to ensure it is priming and that there are no leaks or other problems. This makes it easier and cleaner to work on, identify and properly rectify any issues before moving to paint.
Avoid airless valve sticking and not priming problems in the future by following the guide to storing airless paint spray machines.