Tiny bubbles under sanding sealer

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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.

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.

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.

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.

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

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...

----------------------------------- That's why the Preval was invented.

Perfect for what you are trying to do.

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).

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

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-

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.

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.