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I applied a coat of dewaxed shellac (Bulls Eye) as a first coat on some poplar.

I then noticed that the can said do not apply if humidity exceeds 85%. I live in a generally humid area. Today is a rainy day and the humidity is 95% (89-100% forecast, temp ~60F/16C, indoors no heat), it's one of those days where the books on my shelf are damp. I sanded the poplar heavily first because it had surface moisture and the grain had risen.

My question is: Why can't I apply the shellac at humidity greater than 85%, what happens if I do, and if it was a big mistake how can I fix it?

It definitely is taking a long time to dry. I intend to coat with stain (Varathane, oil base) then a polyurethane (Garco, oil modified) eventually.

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    Why are you shellacking before stain goes on? Normally stain goes directly onto bare wood (with the exception of blotch-prone woods of course).
    – Graphus
    Oct 25 '15 at 20:20
  • @Graphus All I know is it was the nicest looking sequence on my test boards. The stain still seemed to stick, at least enough to give the color I wanted. I probably will use it more appropriately in the future.
    – Jason C
    Oct 25 '15 at 23:32
  • Sorry if you've said and I'm not remembering it, what kind of stain are you using?
    – Graphus
    Oct 26 '15 at 9:27
  • @Graphus Various colors of Varathane brand soy oil based stain. But it has NANO PIGMENTS (jazz hands) and a guarantee of satisfaction, so I should be good.
    – Jason C
    Oct 26 '15 at 14:30
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    LOL Oil-based stains are more coatings that sit on the wood than penetrating stains in the more conventional sense, which dye/stain the wood fibres themselves. So it is very easy to mar the colour produced or remove them from the surface entirely with a little too much rubbing, before the final finish goes over them to sort of lock them in place and add a protective coating.
    – Graphus
    Oct 26 '15 at 17:48
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Why can't I apply the shellac at humidity greater than 85%

A few reasons.

One is that a very high water content in the air can slow drying, as you've experienced (a thin coat of shellac would normally take no longer than 10-15 minutes to dry) or prevent proper drying.

The main reason though is that as alcohol evaporates from a surface it cools it — just as you can feel if you get some on your fingers — and as the alcohol evaporates out of the shellac coat it can chill enough that it actually condenses moisture out of the air onto the surface. Shellac is moisture-sensitive as you probably know and this condensed moisture can be enough for the shellac to go cloudy. This clouding can sometimes be fixed, but obviously it's better to avoid it if possible.

But, as with most instructions on finish labels the 85% humidity thing should be taken as a general guideline rather than an absolute. I regularly apply shellac at humidities above 80% because I have no choice: it's always that humid here. The reason this can work is that relative humidity doesn't directly correlate to an absolute amount of water in the air, it's tied to the temperature. So it's much much more moist in the tropics at 100% humidity than it is somewhere more temperate. Also, how heavily the shellac is applied is a factor since the thicker a coat is the more alcohol is evaporating and the cooler the surface can get. So thin wiped-on coats can work without a hitch even when it's very humid.

Now that said, it is probably a good idea not to use shellac on the wettest of days. It's a pain, but a product's limitations are what they are and we have to work within them.

and if it was a big mistake how can I fix it?

It's only a mistake if it caused an issue. If your shellac eventually dried right — hard, not sticky and with no clouding — then you're fine.

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  • ... And the how can I fix it part: You could probably re-sand it, and that should fix it.
    – Cullub
    Oct 27 '15 at 23:50
  • @cullub, well you can sand it off but as shellac is permanently soluble in alcohol it's much better to wipe it off using the solvent. Sanding off dried finish should anyway be reserved for when it's the only viable method ideally. Scraping is far preferable, or stripping.
    – Graphus
    Oct 28 '15 at 9:50
  • Some sources suggest a light wipe of alcohol to eliminate cloudiness. The mechanism is a function of releasing the water during the alcohol's dissolving of the shellac. Using a fine mist sprayer without touching the surface would work best.
    – HerrBag
    Jan 27 '17 at 19:23

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