Why did my polyurethane varnish bubble/crinkle?

Question

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.

Answer 1

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

I'm thinking this may partly stem from the use of the sealer. 'Sealers' can be any of a number of very different things, some of which you wouldn't want to use under varnish.

But notice the directionality of the streaks of wrinkling appear to follow grain features. This suggests that something related to the acacia is contributing to the effect, or may be the actual cause.

Regardless of the underlying cause my guess about what's happened is that the second coat of varnish softened the first, semi-dry, coat enough that it swelled and released from the wood surface in places where it adhered less well.

I don't think the varnish itself is faulty, given that it is apparently drying well enough for you to be able to sand it down (presumably without it gumming up your paper).

If you need to continue to add poly to increase coat thickness, or to even up the surface finish I would suggest thinning it slightly (adding 1/4 to 1/3 solvent) and adopting part of the procedure for wiping varnish — wiping off most or all of the excess varnish after application. Note that the varnish doesn't have to be wiped on — wiping varnish never needs to be applied by wiping, although you can if you like. You can pour, spray, brush or roller it on if preferred.

---

applied the first coat of varnish, seemed to be fine, waited 24h (recommended recoat time is 12h)

Always remember that recoat times are only a suggestion, often based on very idealised conditions. Where I live there might be 10 days in a year when local conditions match those the instructions are based on!

For most of us, very occasionally you can recoat faster than indicated. But it's far more common for users to have to wait longer than the instructions suggest, sometimes a lot longer — double or triple the time indicated (and in winter even longer than this).