Wagner 404 Plus Airless motor stops.

If you have an airless sprayer yes , you can thin with a small amount of water or use flotrol for latex available at home depot or other box stores. If you have hvlp system no you can't. The hvlp uses a turbine wheel to compress air to around 3-5 psi. You did not say model Wagner sprayer you have. I use my Wagner to 1800-2000 psi. If the job is small you will have to consider the clean u time usually abound 30-45 minutes. Good luck

I haven't much experience with airless sprayers but I have heard the bad language from those who use them.

I expect there is still paint in the business end of the mechanism and an air bubble stuck between that and the new paint fill.

I think the sprayhead will have to be removed or dismantled so the air will be able to escape and allow priming to take place. I suggest spending a little time with the instruction manual.

Your Wagner 1250 is a diaphragm airless sprayer, which uses hydraulic oil to push the diaphragm, so the oil must be hydraulic quality as it gets quite hot; don't use any other type. The Wagner part number for their quart size is 0088009

The Wagner Paint Crew airless sprayer does not require oil for its operation like full professional sprayers. The piston is off-set from the pump chamber with the inlet and outlet valves opposite each other, however you need to protect the parts from corrosion between use and Wagner recommend running some light oil through it after cleaning. If you will be using it in a day or so for water based paint, you could run a 10% solution of their EasyGlide through it.

YOU HAVE A STUCK INLET BALL
Have the unit in water, pressure relief valve turn the machine on and tap with a hammer at the inlet ball, This frees it up 90 of the time.
Otherwise the machine need to be disassemble

Most airless spray guns spit a bit when you pull or release the trigger because the paint between the spray tip and the paint valve is under pressure but not enough to atomise the paint. If you increase the pressure you may reduce the spit to a point when you don't notice it. A blocked filter could slow the paint getting through to the tip cause lower pressure.

The Wagner Capspray 9000 HVLP is about 20 years old, but you should be able to find parts, at least for the HVLP gun. The manual for the turbine is #0276989. If you are in the US try Scott at American Airless Wagner Titan Graco Paint Sprayers or in Europe try us at:
Home Lion Industries

http://lib.store.yahoo.net/lib/gleempaint/404Plus-manual-english.pdf

http://lib.store.yahoo.net/lib/gleempaint/404plus-parts.pdf

I cannot get the sprayer to prime or spray

I worked at a tool rental center when I was going to college and learned many useful tips / tricks about airless sprayers. Airless sprayers have two check valves/balls that can stick on their seats or stay open due to chips of paint on seat. Easy method to prime is to remove intake filter from machine, tilt the machine back far enough so that the pick-up tube is pointing up high enough to pour water in tube until full; hold in this position and turn on machine. If water is sucked in turn off machine and tilt machine into pail of water and restart to check for prime. If it still will not prime remove water from pickup tube and use air pressure into tube with bypass valve open. Air pressure will attempt to release the stuck lower check ball. If air pressure does not release stuck ball you may have to use physical pressure against ball. If lower check ball was stuck you can assume that the upper check ball is probably stuck also. An aid to loosen stickey valves is to pour solvent into pickup tube with machine tilted back as before leave overnite. Enjoy