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, tannins and especially resin in knots are a problem and it is very hard to seal everything inside the wood. Over time, especially in warm periods, the resin expands and tends to come out through the finish layers. 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 that 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 sealing knots.

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 years after the furniture has been made. Finishing can be done without any sign, but after a while a yellow stain may appear at the knot. That is why the solution is often to remove the knot and replace it with thin twigs that mimic the structure of the knot. And if the knot is falling, it is mandatory to replace it because drying will loosen the wood and it will fall off.

But there are situations where the presence of nodes is an asset. In this case, solutions must be found to minimise 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 it is the most recommended. It can be used transparent or coloured white or black.

The shellac knot is blocked with the following steps:

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

Shellac does not resist to the outside. If it has been used for blocking knots in fences, pergolas or other objects used outdoors, it should be coated with outdoor resistant paints.

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

Other products recommended for sealing knots are those that are commonly used to block burning or cigarette smoke odours. 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 may appear in the film 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.

The water-based primer is an acrylic dispersion. It is best to thin it to penetrate as deeply as possible into the wood. As the acrylic molecules in the dispersion dry, they come together and polymerise to form a very strong product. It is like water-based varnishes that do not crack like solvent-based varnishes when a weight falls on them. Neither does the resin in the knot crack and 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 has adhesion.

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

Allow the necessary drying time according to the manufacturer's recommendations, then remove the excess with a chisel or, even better, a rhindea. This is the best way to bring the node level with the rest of the surface. By sanding, you can push too hard in that spot and a bump can occur.

To make the knot look as natural as possible, wood dust can be added to the resin mixture. Black dye can also be added to the resin for knots with large cracks. At the end the resin will appear as a black stripe 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 with resinous, pine being the "champion".

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