Tiny bubbles under sanding sealer

Dear Experts,
My efforts continue to create a glossy lacquered finish on a piece of black walnut.
After my previous experiments it became clear that I would need to fill the grain with something before applying lacquer. To verify this I tried using some "natural" grain filler, and the result was nice and smooth and quite glossy:
http://chezphil.org/tmp/grain_filler.jpg
But for the real thing I needed something transparent, as the presence of the "natural" grain filler was very obvious. So I bought some clear cellulose sanding sealer. After 3 coats of this the wood was looking quite good, from a distance and in the right light:
http://chezphil.org/tmp/dist_view.jpg
I was hoping that I would be able to sand that and then spray lacquer. Unfortunately, from close up and especially after sanding, many small bubbles are visible in the grain:
http://chezphil.org/tmp/bubble_closeup.jpg
These bubbles weren't evident immediately after applying the sanding sealer. It's possible that I wasn't looking carefully, but I also wonder if air from deeper in the grain has risen while it was drying. Or perhaps it is because I had wiped the wood with white spirit ("mineral spirits") beforehand, and its vapour has come out of the wood under the sealer.
Any suggestions, anyone?
Thanks, Phil.
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Phil Endecott wrote:

That could be. ___________

Could be if your "white spirits" actually were mineral spirits (AKA "paint thinner" in the US) and not alcohol. If it *was* alcohol, I can't see it causing a problem as it doesn't leave a residue as does paint thinner.
To mitigate your problem, I would spray on a fairly wet coat of *lacquer* thinner - assuming that is the thinner for the sanding sealer - so it can melt the bubbles. When completely dry, use more sanding sealer if needed; if not, go to regular lacquer.
--

dadiOH
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UK "white spirit" = US "mineral spirits" = comes from oil UK "methylated spirits" = US "de-natured alcohol" = ethanol

That's an interesting idea. I have some cellulose thinner in a bottle, but no good way to spray it. I'll try to improvise something; the worst that can happen is that I have to sand it again.
Thanks, Phil.
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"Phil Endecott" wrote:

----------------------------------- That's why the Preval was invented.
Perfect for what you are trying to do.
http://tinyurl.com/3qactu9
Lew
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wrote:

That might work.
Another question is "Does it dissipate or show with a finish added over it?"
-- Happiness lies in the joy of achievement and the thrill of creative effort. -- Franklin D. Roosevelt
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wrote:

It's not very visible until I sand it. My feeling is that it will remain clearly visible if I now lacquer it. Perhaps if I now apply more coats of sanding sealer, then sand, then lacquer it will be harder to see. But I will still _know_ that it's there...
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On Sat, 15 Oct 2011 13:43:54 -0700 (PDT), Phil Endecott

Oh. We wouldn't want invisible blems in our projects, would we? :^)
-- Happiness lies in the joy of achievement and the thrill of creative effort. -- Franklin D. Roosevelt
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Phil Endecott wrote:

Your sealer is quite probably lacquer with talc in it. If you scratch a piece of talc, you get a white scratch mark but once wet again the white disappears.
In my previous incarnation I was a photographer. In order to retouch color prints they need a "tooth". That tooth could be applied via spray cans of retouching lacquer but they weren't cheap; always ascribing to the "a penny saved is a penny earned" adage I started making my own - talc in lacquer. Upon spraying same, I would sometimes get very small, white flecks where the spray had built up around a piece of dust or the like. They disappeared when sprayed with regular lacquer after retouching.
--

dadiOH
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First guess would be out-gassing while drying.
Beyond making sure all volatiles have dried, and testing for chemical compatibility before sealing something, temperature control will play an important part in fine finishing.
The trick is to spray while the surface is cooling - not warming.
Carefully note the "ing"s there. I'm talking about the temperature of the surface changING - not a static temperature.
If the surface is getting warmer while the coating dries/cures then you often see tiny gas bubbles form in the varnish/epoxy /sealer/etc.
On 10/15/2011 10:29 AM, Phil Endecott wrote:

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That's possible; I have mainly been doing this outside, for ventilation, and temperature will have changed significantly over the course of the day and night.
So perhaps if I keep it inside in the warm, then take it outside to apply the sealer mid-day, and leave it outside for the afternoon/ evening to dry, I will get better results.
Thanks for the suggestion.
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On 10/15/2011 1:55 PM, Phil Endecott wrote:

Mine works the other way - outdoors to warm it up, and bring it in to varnish.
But being aware of which way the temperature is going seems to have been the trick!
Best of luck, Phil.
PS- http://www.home.earthlink.net/~sv_temptress/refine.htm
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Phil Endecott wrote:

Was this wood previously finished? If so, you might be seeing a reaction to the residual finish lodged in minute dents in the wood. One solution is to sand the heck out of the surface to get the surface below even the tiniest dent. Another, easier, solution is to wipe the surface with a series of sovents. My table did the same thing until I hit it with Xylene (this was after Mineral Spirits, paint remover, alcohol, and anything else I could think of).
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Or seal with dewaxed shellac or shellac sanding sealer. before refinishing and after fully sanding. Use 1 lb coats or less. a few build ups.
On 10/15/2011 5:52 PM, HeyBub wrote:

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The sanding sealer is attacking the filler because it has a more active solvent. Try pumice as filler next time. Apply a couple of spit coats of dewaxed shellac (lacquer won't stick to wax), sprinkle with 4F pumice, then rub in with an alcohol-dampened pad.
Once the pores are filled, you really don't need a sanding sealer.
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