I'm covering an acacia board with an oil-based polyurethane varnish. I've used the same brand of varnish, and even the varnish from the same can, on different types of wood before without a problem.
I've sealed the board with an oil-based sealer before applying the varnish (same brand), waited 24h before applying the first coat of varnish
applied the first coat of varnish, seemed to be fine, waited 24h (recommended recoat time is 12h)
lightly sanded the surface
applied the second coat. After ~5 min the surface started bubbling/crinkling like this:
- after drying, it now looks like this:
The problem happened only in a couple of places of the board, the rest of the desk top is fine.
I've sanded the problem areas and applied another coat. The problem happened again in a slightly different spots.
What would be the reason for this problem and how do I avoid it?
Update: Looking at the board I can see that the defects exclusively (but not always) occur in the areas of the dark wood, which is very visible on the photo below:
So it seems that the problem may have something to do specifically with the properties of the acacia wood, although I'm not sure what could it be.