Bubbles on Melamine lacquer

[Rhossydd]

I haven't seen this problem before;

Air dried English Ash, kept indoors for the last six months.
Finished cutting and sanding last week and started final finish.
Two coats of Chestnut Cellulose sanding sealer, thinned with 30% thinners applied with a foam brush. One coat of Chestnut Melamine lacquer all left to dry for at least 24hours before de-nibbing and re-coating. This morning a final coat of Melamine lacquer thinned with 20% thinners and these bubbles appear over two small pin knots.

Tricky things to photograph.
I assume this has something to do with it being rather warm when putting on the lacquer and somehow trapping air within the knot ?
Never had this problem before, has anyone else ?

 

[Richard S]

I think your problem is solvent entrapment, I've had it using pre-cat lacquer, usually as you say when it's warm. The problem is caused because the surface starts to dry too quickly and traps the evorating solvent beneath it. When I get it it is usually much worse than this. I don't know if the knots are relevant, maybe soaked up more finish than the surrounding area, although open grained timber such as ash or oak tend to suffer from this problem around the open grain structure. With just a couple you may get away with a quick flick over with some very fine wire wool lubricated with a little turpentine.

 

[Rhossydd]

I think your problem is solvent entrapment, I've had it using pre-cat lacquer, usually as you say when it's warm. The problem is caused because the surface starts to dry too quickly and traps the evorating solvent beneath it.

Sounds a possibility. Especially as it was only the thinned lacquer coat that has shown this issue.

I don't know if the knots are relevant, maybe soaked up more finish than the surrounding area,

I'm sure they are as it's only over the two tiny knots on one piece that this problem has occurred. I've finished 48 other, knot free, components without any issues.
I also wonder if the lacquer, which is relatively viscous and fast drying, was forming a skin on as it was being laid and trapping air in the knots, rather than flowing into the tiny voids ?

 

[Richard S]

That's exactly what's happening except it's not air that is being trapped but the gas given off by the solvent in the lacquer as it cures, as well as being known as solvent entrapment it is also referred to as off gassing.