Why do Knots Occur in Timber? - Armstrong Cheshire - Armstrong Cheshire
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Why do Knots Occur in Timber?

Why do Knots Occur in Timber?

Sarah Macklin |

 

Have you ever owned a piece of wooden furniture and noticed dark circles. You may have even tried painting the wood and found that these circles bleed through the paint. They are called wood knots and look like this:

The knot is formed when a branch has fallen off the tree. This branch may have died and fallen off, or it could have fallen due to natural causes, such as strong winds. Where the branch has detached, the tree continues to grow around it as its trunk expands over time. Knots don't usually affect things that are made of wood, such as furniture or floorboards. They may change the wood's appearance but don't impact your furniture's structure. However, timber with a lot of knots may not be suitable for applications where it bears a lot of weight (such as the beams in a house) because it can be structurally weaker.

The two most common types are tight knots and loose knots. However, there are other kinds too, which we'll discuss further. 

Tight knots form when a branch becomes embedded in the trunk as the tree grows. The branch is still living, so the branch and trunk continue to grow together. Tight knots are dense circular spots with an irregular grain. This kind of knot is integrated into the wood and won't fall out if you sand or saw the timber. 

Loose knots are when the branch has died and is embedded in the tree. When this wood is cut for timber, the knot can easily fall out and leave a hole. It is these knots that reduce the strength of the wood.

The other types of knots that you may find are called epicormic sprouts and are caused by excessive pruning. Epicormic buds lie dormant under the tree's bark. Their growth is suppressed because the tree's energy is put into growing active shoots higher up the tree. If a tree has been excessively pruned, buds and branches lower down get more light, which can make these dormant epicormic sprouts active. These epicormic branches should be removed, otherwise lots of knots can form in the wood. 

 

How to treat knots in wood

If your timber has knots, there are some steps that you need to take to ensure they are secure and won't fall out. 

First, identify if they're tight or loose knots. Loose knots will have lots of cracks and gaps around the edge, and you may be able to feel them move if you apply a small amount of pressure. Tight knots will look like they've already been sanded into the wood. They should be smooth and shouldn't move at all. 

Once you've identified your loose knots, you should use a small amount of clear epoxy resin to fill in the gaps around the knot and ensure that it won't fall out. Once the resin has dried, you'll be able to sand and cut your wood without worrying about the knot falling out. 

 

How to stop wood knots from bleeding through paint

Wood knots are renowned for bleeding through paint, and it's a common problem amongst DIYers, upcyclers and painters. This is because there's more sap in a knot than in the rest of the wood. This sap will bleed through the paint and leave a circular mark. 

Knots can add charm and interest to a piece of furniture if you aren't painting the wood.

However, if you're painting the wood, you don't want the knots to bleed through. To prevent this from happening, you should start by sanding the wood down so the surface is smooth. Then, clean all the dust off the wood using a damp cloth. Allow drying for around an hour. 

Once the wood is dry, you should use a wood primer. Apply it to the whole unit and not just the knot. The primer acts as a seal and closes the pores of the wood. It also prevents the wood from absorbing the paint, so you may not need to apply as many layers. 

Once the primer is dry, and you're happy that the knots are sealed and haven't bled through, you can paint the unit. 

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