I agree with the below comments and would add: 

     The thicker the resist viscosity and coating weight, the greater the 
     possibility of bubble retention. The lower the temperature, generally 
     the thicker the viscosity.  Fast stirring will fold air into the 
     system so stir as slowly as possible but long enough for complete 
     mixing. It's generally best to stand before use to let trapped air 
     come out over a period of an hour or so.  Perhaps more to the point... 
     
     Talk to your supplier.  
     
     LPISM suppliers have tech support staff to whom problems like this are 
     bread and butter.  Viscosity control is sensible even just before use 
     in cases where little mixed mask is used per shift (some masks thicken 
     on storage) but this might not be the whole answer.  Part of the job 
     of formulating LPISM's involves additives to reduce bubbling, 
     reticulation etc.  Your supplier will know about bubbling and should 
     be able to help.  He might even be able to recommend a minor change in 
     resist (viscosity, thinning solvent etc) to meet your particular 
     requirements.
          
          
          Bob Courtney
          Coates Circuit Products


______________________________ Reply Separator _________________________________
Subject: FW: Ink preparation mixing bubbles
Author:  [log in to unmask] at Internet
Date:    07/05/96 10:56




My experience with bubbling has been that the viscosity of the ink is   
most critical.
Mixing will always inject air as does forcing the ink through a screen.
The key factors are; how viscous is the ink (It's hard to get bubbles out   
of peanut butter.),
how thick is your coating (thicker coatings are harder to get the bubbles   
back out of), and how long do you allow product to sit for de-bubbling   
before tack-dry at what temperature.
We had a rash of bubbling problems recently when one of our two suppliers   
sent in some thicker than normal ink.  We're looking at incoming   
inspection for viscosity.