Tiny Bubbles, Not In My Wine...

[Derek Arita]

I don't have a lot of experience spraying, so bear with me...sometimes when I spray on a fairly thick coat of waterborne poly, tiny bubbles form in the finish. When I look at them with magnification, they almost look like they can be attributable to raised grain, although a sanding and reshoot helps, but doesn't always fix the problem. I have been thinking that in an effort to lay down a fairly thick, leveling coat, I've been laying on too thick, causing the bubbling. Just for info, I try to adjust my gun to spray with as low a pressure as possible, with a medium spray pattern and medium amount of material output.

[Art Mann]

This is mere speculation but people have described a similar problem when laminating with epoxy. If the day is getting warmer, then the temp rise will sometimes cause the wood to out gas. The solution to that problem is to apply while the temperature is decreasing. In this case, the wood will tend to pull the material into the pores of the wood instead of expelling air.

[Derek Arita]

This is mere speculation but people have described a similar problem when laminating with epoxy. If the day is getting warmer, then the temp rise will sometimes cause the wood to out gas. The solution to that problem is to apply while the temperature is decreasing. In this case, the wood will tend to pull the material into the pores of the wood instead of expelling air.

Interesting you should say that, as it's been unseasonably warm here in NorCal lately. I've been finishing in the mornings, however the temp is rising from the low 70s to the 90s. The wood however, has been in a garage, with temps remaining in the 80s all night, so my finishing outdoors in the shade may even be a tad bit cooler when I begin. That said, I'll keep that in mind.

[John TenEyck]

If you are putting the WB directly on the raw wood then the water in the finish might be causing the grain to raise. This is a real and sometimes big problem on coarse grained woods like oak, ash, etc. There are several things you can do if that's the problem.

1) Sand to a finer grit before applying finish, and make sure to use fresh sand paper for the last grit. You want to cut the wood fibers off cleanly and only fresh paper will do that. I sand oak to 320 grit when I want to use a WB topcoat directly on the wood, whereas I normally only wand to 150 or 180 grit when using oil based products.

2) Wet the wood with distilled water to pre-raise the grain. Just wet it with a sponge dipped in water until the surface is good and wet. Let that dry at least over night and then sand very gently with 320 or 400 grit to cut off those nibs that formed. Two or three strokes with the paper on a rubber block should do it. If the grain is raised so bad that you have to sand more than that then you should definitely sand to a finer grit first.

3) You can eliminate most or all of the above if you seal the raw wood with Sealcoat shellac before applying your WB topcoat. Shellac will raise the grain only slightly, easily dealt with with a few strokes of 320 or 400 grit sandpaper, and then you can apply your topcoat w/o any further problem. This is my preferred approach unless I don't want the ambering effect that shellac imparts to the wood.

John