I just posted this same question, you cant print water based on top of plastisol, you can do the reverse.

For screen opener check this
http://www.t-shirtforums.com/screen-printing/t16345.html

My personal experience is that waterbased is very limited.

 

if anyone cares...this is what i ended up doing.

first i tried to completely remove the water based from the plastisol using water and dish soap without much success. the water based would not completely wash away. after i realized this would take me forever i decided to print black plastisol over the water based black. it took a while to line the screen up with the already printed shirt but i did it fairly well. i figure that any place the two inks don't overlap will be fine since the water based will just wash out.

i think my headache has finally subsided.

 

To keep ink from drying in your screens try flooding the screen with ink as in running our squeegee through very lightly so the ink simple sits on top of the stencil without going through it.
This can be a problem with waterbased ink due to it's viscosity.

Another factor is your platen temperature. If, while printing, your platen gets too hot, the ink in your screen will begin drying as it comes in contact with the shirt sitting on top of the platen.
To solve this you can adjust the power on your flash dryer, if you don't have a power adjuster, I got mine from Rjennings. I don't think it's listed on the website, but just call and ask, it's pretty cheap.
Another solution is to print wet on wet. Simply spray the print side of your screens with silicone spray, that prevents the wet ink transferring on the back and ultimately on other shirts.

One more thing to consider is a well ventilated workspace with a cool temperature.

Hope that helps! Peace, Salam.

 

add the retarder to your white ink and mix well. it will slow down the drying of the water base ink.

 

Good advice.
I am assuming that you are using a discharge ink! We will often mix wilflex blending white with the super white and get good results. Water based inks are finicky, the humidity in the air, air temp and other factors can influence this ink and how it prints. If you are printing manually it is probably taking too long between stokes and the ink is drying. Wilflex sell a retarder that will help but it is not a cure all. Good luck.