Ways of blocking resin and tannins in wood knots

I was asked if the knots could be sealed with shellac. I replied that, as far as I know, the tannins and especially the resin in the knots are a problem and it is very difficult to seal everything inside the wood. Over time, especially in warmer periods, the resin expands and tends to come to the surface penetrating the finish coats. This is especially noticeable if the finish is white or another light color. A disturbing yellow stain will appear at the knot. How did we know shellac is the wonder lake that has been used for hundreds of years to protect wood I promised I would try to find out more. So I spent some time trying to find out as much as I could about knot sealing.

First of all it is good to know that nodesalthough they are beautiful and give a special charm rustic furniture, can be a source of problems even years after the furniture has been made. Finishing can be done without any sign of a yellow stain appearing after a while. That is why the solution is often to remove the knot and replace it with thin twigs that imitate the knot structure. And if the knot is dropping, it is mandatory to replace it because drying will cause it to detach from the wood and fall off.

But there are situations where the presence of nodes is an asset. In this case solutions must be found to minimize future problems. The solution most often recommended is indeed shellac, both pure shellac made from resin flakes and alcohol, and shellac mixed with other substances. There is also a variety of shellac used as a first coat - primer - and this is the most recommended. It can be used clear or colored in black or white.

Blocking the knot with shellac is done in the following steps:

• Sand the knot very well, following the grain of the wood, with fine abrasive paper or sponges, then sand the surface very well:
• apply shellac with a brush over the knot, insisting that it gets into all the cracks;
• leave to dry for at least 3 hours, then sand and apply another coat;
• leave to dry until the next day, then sand very well and apply the desired finish.

Shellac is not outdoor resistant. If it has been used for blocking knots in fences, pergolas or other objects used outdoors, it should be covered with exterior-resistant paints.

I told you that it is the most recommended method for blocking knots, but there are also many voices that say that if the knot has a high resin or tannin content, shellac does not hold. Over time yellow stain will appear on the knot.

Other products recommended for sealing knots are those that are commonly used to block burning or cigarette smoke odors. The best such materials are oil-based and water-based primers. The oil-based ones are alkyd, those linseed oil-based primers (paints) used in the old days to paint anything, inside and out. They seem to be a good way to lock the knot. With the caveat that the next coat may not be compatible and cracks in the film may appear like the krackle effect. The material used on top should be on the same base or water based. Organic solvent based ones usually have this incompatibility problem.

Water-based primer is an acrylic dispersion. It is best diluted to penetrate as deeply as possible into the wood. As it dries, the acrylic molecules in the dispersion come together and polymerize to form a very strong product. It's like water-based varnishes that don't crack like solvent-based varnishes when a weight falls on them. Neither does the resin in the knot break or crack easily, preventing the tannin from coming to the surface. Any kind of finish can be applied on top provided the knot is well sanded so that the varnish will have adhesion.

The most recommended is the two-component epoxy resin. To apply the knot, sand the knot as for shellac, sand thoroughly, then pour the resin mixed with the hardener onto the knot.

Leave to dry as recommended by the manufacturer and then remove the excess with a chisel or, better still, a rhindea. This is the best way to bring the knot level with the rest of the surface. With sanding, you can push too hard in that spot and cause a bump.

To make the knot look as natural as possible, wood dust can be added to the resin mixture. Black colorant can also be added to the resin for knots with large cracks. At the end the resin will appear as a black streak in the knot, just like natural knots.

I hope the methods mentioned will help you avoid problems you might have from knots. And one last mention: most problems occur at resinous, pine being the "champion".

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