We may receive a commission when you use our affiliate links. However, this does not affect our recommendations.
Because stains and finishes sometimes dry slowly.
A friend called with a problem. He had applied an ebony oil stain to the oak and after the stain had dried for two days, the polyurethane he had brushed picked up some of the color and smeared it on the surface.
Was there a problem with the stain or was there something wrong?
Well, I could think of several possible problems with the stain. First, a very dark spot requires more pigment. So maybe the manufacturer didn’t add enough binder (oil or paint) to wrap all the pigment well. Second, some manufacturers are replacing the solvent with slow-drying oils to comply with California’s VOC rules and then sell this product to the entire country to avoid having to do two lines. Oils dry more slowly.
But my first thought was time. Although it has been mild lately, with highs in the 60s and lows in the 40s and 50s, it is still too cold for normal drying. Most stains and finishes require a temperature of at least 65 ° -70 ° for eight hours or more to dry at a normal speed. (Exceptions are water-based stains and finishes, which are more affected by humidity than temperature; and lacquers, which can be dried normally in cold weather by adding acetone or fast-evaporating paint thinners available in stores. of supplies for bodywork.)
So I asked about the store temperature as the stain was drying. My friend assured me it had heat, but after further questions he revealed that it turned it off at night.
After personally testing the stain in my hot shop, I concluded that the problem was a lack of adequate heat. This is usually the cause of stains and finishes drying slowly, and is the first thing you should think about when faced with a drying problem.
If you can’t avoid a cold store, you could bring the project indoors after each coat, or you could keep the finish warm with a heat lamp or even a small item hair dryer. Alternatively, you’ll just have to let each coat dry longer.
Temperature isn’t the only cause of slow drying. Others include:
• Application of oil or paint on oily woods;
• Do not remove all excess finishing oil; And
• Using too old shellac.
Most exotic woods, such as teak, rosewood, cocobolo and ebony, contain natural resins that touch and act like oil. These resins delay the drying of oils and paints (and even oil stains).
This is counterintuitive, so much so that it is common to see instructions in woodworking magazines that specifically request the use of oil or paint on oily woods because these finishes are “compatible”. The reverse is the case. Oils and paints are the only finishes that don’t dry well.
The explanation is this: the non-drying oily resins on the surface of the wood mix with the wet oil or paint and keep the finishing molecules separate so that they do not collide with each other and cross-link. The resins act as paint thinners that do not evaporate.
Once you have applied an oil or varnish to an oily wood and found that the finish does not dry out, there are only two good solutions: applying heat to the surface to excite the molecules so they are more likely to bump and crosslink, or undress the end and start over. It is usually quite easy to remove (actually just wash off) an oil or paint that has not dried using naphtha or lacquer thinner. Sometimes white spirit is strong enough.
To prevent a drying problem before it occurs (or after removing a non-drying finish), remove oil from the surface of the wood or seal the wood with shellac.
To remove the oil, scrub with one of several solvents: white spirit, naphtha, alcohol, acetone or lacquer thinner. White spirit and naphtha are the least effective, but they do not raise and smear the color of the wood. Any of the other solvents may remove some of the color along with the oily resin.
So, if you’ve joined two or more woods, such as in a cutting board or segmented bowl, try white spirit or naphtha first. Before using one of the other solvents, check that the colors do not get dirty on the waste wood.
Whatever solvent you use, wet the wood well with a rag, then wipe it dry with another to remove the oil, not just to smear it. Apply the new finish immediately after the solvent evaporates, so that the oily resin of the wood does not have time to rise to the surface.
Alternatively, you can seal the wood with another finish before applying the oil or paint. Shellac is the most effective at blocking oil.
The common instruction for applying oil finishes and oil / paint blends is to wet the surface well and then wipe off the excess after the finish has had a few minutes to absorb. This statement is vague because different interpretations can be given to “clean away the excess”.
What is meant is ALL excess. The surface should not be left wet to the touch. The oil does not dry well, so leaving even a thin, moist finish film (anything that gets your fingers wet or feels sticky) will result in a sticky surface for a long time.
If you have a situation where you haven’t removed enough oil and it’s now too sticky to wipe off with a dry cloth, follow the instructions above to treat an oil or paint that won’t dry on oily wood.
Shellac deteriorates much faster than other finishes. Deterioration results in slower drying and reduced water resistance.
In solid flake form, bleached or “blonde” shellac deteriorates much faster than unbleached shellac, sometimes within a year or two (unbleached remains good for many years). The variables are the methods used to whiten shellac and the temperature in which shellac is stored, with higher temperatures leading to faster deterioration.
You can slow down deterioration by storing shellac in the refrigerator.
Once shellac is dissolved in alcohol, all types of shellac deteriorate, again faster the higher the temperatures the shellac is stored in. You should use shellac within one year of dissolving it in alcohol if you are using it as a finish and not just as a sealant under another finish.
For any critical project, like a table, you should melt your flakes and use shellac as quickly as possible, within several weeks or months.
Other finishes also deteriorate, of course, but deterioration does not lead to significantly slower drying. Pre-catalyzed paint loses some of its durability after a few years (time varies by manufacturer) and water-based finishes sometimes curdle after a certain number of years. I have never seen paint or lacquer deteriorate as long as the air is kept out of the can, no matter how old.
Here are some supplies and tools that we find essential in our daily shop work. We may receive a commission from sales reported by our links; however, we have carefully selected these products for their usefulness and quality.