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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:

wet varnish

  • after drying, it now looks like this:

dry varnish

  • 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:

dark wood

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.

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  • Have you used this sealer before varnish before? I'm also curious why you used it, since varnish doesn't require any sort of sealer before it's applied — the first coat of any finish in effect acts as a "sealer" coat.
    – Graphus
    Jan 9, 2023 at 7:26
  • @Graphus: I did use this sealer with this varnish before on a different wood without a problem. They're the same brand and on the tin it says that the sealer can be used before the varnish. The reason I used it is because without the sealer the first layer of that particular varnish is getting ruined - the liquid part of the varnish is getting absorbed into the wood leaving a powdery white residue on the surface instead of a film, so I need to sand it off.
    – Sergey
    Jan 9, 2023 at 22:00
  • That is a strange behaviour for varnish. I presume it is a reduced-sheen varnish yes? If you intend to apply multiple coats of varnish to a project it's best not to use a satin, semi-gloss or matt varnish throughout but instead to begin with gloss varnish which doesn't have any of the matting agent in it. This is mainly intended to reduce a sort of clouding or whitening effect on the features in the wood, but it will also help eliminate issues with the matting agent separating out.
    – Graphus
    Jan 10, 2023 at 4:39
  • @graphus: Yes, it's a matt varnish. Using gloss varnish for the bottom layers is an interesting idea which totally makes sense, I'll try it next time, thanks.
    – Sergey
    Jan 10, 2023 at 5:21
  • I should have thought of this sooner, and this is a potentially silly question, but you did thoroughly thoroughly stir the varnish prior to application yes?
    – Graphus
    Jan 10, 2023 at 13:35

1 Answer 1

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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).

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  • It's summer here in Oz, quite warm :)
    – Sergey
    Jan 9, 2023 at 22:02
  • But, I guess, if the cause of the problem is the softening of the bottom layer then the solution, as you suggest, would be to sand the defect off and wait 2-3 days for the bottom layer to completely cure. I'll let you know how it goes, thanks
    – Sergey
    Jan 9, 2023 at 22:04
  • "wait 2-3 days for the bottom layer to completely cure" Obviously waiting longer could do exactly what you hope, but LOL at 2-4 days for a complete cure – try 2-4 weeks.
    – Graphus
    Jan 10, 2023 at 4:35

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