Troubleshooting An Air-Bubble Problem

      Finishers suggest ways to prevent pinholes and air bubbles when spraying a clear conversion varnish on a porous wood such as red oak. February 19, 2006

I just started using Valspar pre-cat lacquer because that's what my suppliers are selling. I have a small HVLP system and I'm spraying a clear finish on red oak. My problem is that I'm getting bubbles in the grainy areas. In the past that's never really been a problem. Does anyone have any suggestions?

Forum Responses
(Finishing Forum)
From contributor C:
Is this being done over oil-based stain? Or is there no stain? Red oak will hold on to that stain and it takes a good while for the solvent to evaporate. The only time I've ever had that problem over stain was when I hadn't waited long enough for the solvent to evaporate. The bubbles are caused by the solvent trying to get out. Other than that, I'm clueless.

From contributor J:
Some finishers always have problems spraying porous wood like oak, mahogany etc. If your material is not atomizing properly, you'll have an uneven distribution of paint or you'll spray heavy coats causing bubbles and pinholes especially on oaks. Since you are using HVLP, try to reduce your fluid pressure to 30 psi and adjust your air pressure to 35 psi, then be sure that your paint viscosity is 20-22 seconds Zahn #2 and /or 18 -19 seconds on the Ford #4 and try to use 09 tip.

It should spray like a mist. Then you can spray 2 to 3 passes with a distance of 6 to 8 inches. We have tested and proven this procedure producing hundreds of oak and mahogany doors a day. We spray any porous wood material using this application technique with only 2 wet mils and still maintain the sheen.

From contributor R:
I would suggest trying a slow reducer instead of thinners.

From contributor B:
Do you mean "pin holes" or actual air bubbles? Pin holing on oak is common, especially when the solids content of the finish is above 25%nv, or if the coating shares a common polar charge such as a WB. Did you see this effect during the first coat or did it occur after the 2nd and 3rd+ coat?

From contributor S:
Add Butyl Acetate, start with a 2 % add and go up from there. That will open up the film and keep it from skinning over on top long enough to let the air out of the deep pores of the oak and improve the flow of the finish.

The viscosity is set on the Valspar Pre Cat to offer decent build to those that need it. Allow a little more dry time before sanding or rubbing if either of those processes are going to take place. Butyl Acetate is a good lacquer, pre cat, and conversion varnish retarder. Valspar is great.

From contributor T:
I use M.L. Campbell precat and have the same basic problem with tiny pinhole bubbles. What I do is simply spray a wash coat of the finish first (usually 10%) to partially fill the deep pores. After that dries, sand and then spray 2 full wet coats. It's a pretty painless fix. My supplier recommends this in their literature and I've had to use this process not only on oak and mahogany, but also birch (thought strangely alder doesn't seem to need it).

From contributor L:
275 VOC coatings are loaded with acetone (non-HAP, non-VOC solvent) to meet the strict CA requirements. They dry really fast and can cause bubbles if they are applied heavy or if there is any air movement across the substrate. Retarder like MAK (haps compliant) should help quite a bit. Add 4 ounces per gallon to start.

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