Eliminating Micro-Bubbles in Polyurethane when Spraying with an AAA Gun

A few tips on resolving a bubble problem in polyurethane finish when applying with an air-assisted airless spray setup. August 22, 2014

Question (WOODWEB Member) :
I have been working on a finishing job using General Finishes clear satin polyurethane, running it through a Kremlin 10:14 with a CAT tip, 06-114. When using the product straight up with no thinning I had a huge amount of bubbles, almost like solvent pop, which did not level out and gave a very poor finish. I then added 5% of flow enhancer which did not seem to help at all. I thought it was the low humidity in the spray area. I then added 20% water to the mix and that solved the problem. I am thinking that due to the dry climate this winter the finish was skinning off too quickly and I was laying on the coats full 4 mils wet. I went to 2 mil wet and 2 extra coats and achieved a much better result. The pump was set to 35 fluid and 20 air. Hope these notes help someone else.

Forum Responses
(Finishing Forum)
From Contributor C:
With Air Assisted Airless systems like the Kremlin, micro bubbles can come from the piston action on the pump pulling the material through a small pick-up tube. The two changes that have helped people spraying the material with this type of equipment is to switch to a double orifice air cap and change to a larger diameter pick-up tube. This reduces the shearing effect of the product being forced through a small tube. The double orifice air cap breaks the product up into smaller droplets for better atomization.

From Contributor O:
I had a very similar problem with EM2000 sprayed out of a Kremlin 10-14 with MX gun and 06-096 tip. I tried an 09-096 tip but no real improvement. I searched around on the internet and found an old post on this forum where a member cured the problem by upping the fluid pressure to 50psi rather than dilute the finish. It worked.

From the original questioner:
I upped the fluid pressure to 60 and that seemed to help a lot. I had it previously down at 30 with air up at 30. Thanks for the tips everyone.