My cousin sent me a bunch of exotic woods, some known and some unknown. When I started using them for knife handles I ran into problems. On some of them the oil finish I was using (TruOil) would not harden like it did on other handles and like it was supposed to. The problem was that the oils in the woods were doing something that stopped the oil finish from properly curing. I found an article by Jeff Jewitt in the Feb, 2009 Fine Woodworking Magazine (pages 94-95), that explains why this happened and what can be done to prevent it. Below is a summary of what it said.
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The wood
The wood has a higher than normal level of oils in them and the oils have a high level of antioxidants. In nature these oils and antioxidants help prevent rot.
Oil finish
Oil finishes begin to dry and harden by absorbing oxygen from the air into the liquid finish put on the wood. Oxygen in the air combines with the chemical components of the finish. This forms yet other chemical molecules, one of which is a free radical. A free radical is unstable and thus highly reactive because it has too many electrons (or not enough electrons to fill the outer shell). The polymerization (curing) of the finish depends on the acceleration of the reactions caused by the unstable free radical.
Now we introduce the antioxidants from the wood. The antioxidants donate a portion of their electrons to the free radical, (filling its outer shell) stabilizing it and effectively neutralizing it. This can either slow down the polymerization process or prevent it all together.
The Solution - sealing
In the article is says that wiping the wood down with lacquer thinner or acetone prior to applying the oil finish may not work because as the solvent evaporates it pulls more oils to the surface. Rather, sealing the surface of the wood with something that is not affected by the oils is suggested as a solution. The author specifically recommends dewaxed shellac as the sealant to use.
Apparently you can use dewaxed shellac flakes to make your own sealant or buy a ready-made product such as wax-free Zinsser SealCoat. A single coat of it should do the trick. We are cautioned that brushing the shellac on could pick up color from the wood and spread it to other areas causing it to stain. This is a problem when you have two types of wood next to each other as with an inlay. This is not usually a problem with a knife handle, but if it is then spray the shellac on rather than brush it on.
We are told that after the shellac dries we should sand it with P600-grit (400-grit CAMI) sandpaper and continue on with the finish of our choice. The author states that after this single sealing with dewaxed shellac one can use oils, oil-based finishes, water-based finishes, lacquers, urethane, and of course shellac on the wood.
Another alternative given is to not apply any finish at all. Some oily woods (not all) can be given a high shine by simply polishing it with a buffing wheel.
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I wish I had this information earlier. Ive not tried this myself, as I dont seem to have any de-waxed shellac on hand.
From personal experience it seems to me that sunlight and warmth helps a stubborn oil finish harden. I had two handles that would not set-up properly. They were not sealed with anything but were wiped down with acetone to get rid of the oils. They would not set up, or were doing so a an extremely slow rate when kept indoors. I built a gizzmo to hold them in a standing up position and placed them on my deck. A couple days in the hot summer sun would usually do the trick. The finish would harden and then I could sand the handles and apply another coat of TruOil and put them back outside in the sun for another couple of days.
Has anyone experience using shellac as a sealcoat on oily wood they can share with us?
LonePine
AKA Paul Meske, Wisconsin
- - - - - - - - - -
The wood
The wood has a higher than normal level of oils in them and the oils have a high level of antioxidants. In nature these oils and antioxidants help prevent rot.
Oil finish
Oil finishes begin to dry and harden by absorbing oxygen from the air into the liquid finish put on the wood. Oxygen in the air combines with the chemical components of the finish. This forms yet other chemical molecules, one of which is a free radical. A free radical is unstable and thus highly reactive because it has too many electrons (or not enough electrons to fill the outer shell). The polymerization (curing) of the finish depends on the acceleration of the reactions caused by the unstable free radical.
Now we introduce the antioxidants from the wood. The antioxidants donate a portion of their electrons to the free radical, (filling its outer shell) stabilizing it and effectively neutralizing it. This can either slow down the polymerization process or prevent it all together.
The Solution - sealing
In the article is says that wiping the wood down with lacquer thinner or acetone prior to applying the oil finish may not work because as the solvent evaporates it pulls more oils to the surface. Rather, sealing the surface of the wood with something that is not affected by the oils is suggested as a solution. The author specifically recommends dewaxed shellac as the sealant to use.
Apparently you can use dewaxed shellac flakes to make your own sealant or buy a ready-made product such as wax-free Zinsser SealCoat. A single coat of it should do the trick. We are cautioned that brushing the shellac on could pick up color from the wood and spread it to other areas causing it to stain. This is a problem when you have two types of wood next to each other as with an inlay. This is not usually a problem with a knife handle, but if it is then spray the shellac on rather than brush it on.
We are told that after the shellac dries we should sand it with P600-grit (400-grit CAMI) sandpaper and continue on with the finish of our choice. The author states that after this single sealing with dewaxed shellac one can use oils, oil-based finishes, water-based finishes, lacquers, urethane, and of course shellac on the wood.
Another alternative given is to not apply any finish at all. Some oily woods (not all) can be given a high shine by simply polishing it with a buffing wheel.
- - - - - - - - -
I wish I had this information earlier. Ive not tried this myself, as I dont seem to have any de-waxed shellac on hand.
From personal experience it seems to me that sunlight and warmth helps a stubborn oil finish harden. I had two handles that would not set-up properly. They were not sealed with anything but were wiped down with acetone to get rid of the oils. They would not set up, or were doing so a an extremely slow rate when kept indoors. I built a gizzmo to hold them in a standing up position and placed them on my deck. A couple days in the hot summer sun would usually do the trick. The finish would harden and then I could sand the handles and apply another coat of TruOil and put them back outside in the sun for another couple of days.
Has anyone experience using shellac as a sealcoat on oily wood they can share with us?
LonePine
AKA Paul Meske, Wisconsin
