Painting and finishing wood is a very satisfying task when done right, seeing your reflection in the smooth glass-like finish makes all the hard work worthwhile, but if you forget or miss this one simple step your hard work will be for nothing.
Not treating the knots before you apply primer or paint will mean the knots are not sealed and can weep resin, which will damage the painted or varnished surface ruining your hard work.
The best way to treat knots in wood is to use shellac-based knotting solution. Brushing a thin layer over the area where the knot is visible will seal the wood and stop the resin from weeping and damaging the surface. Knotting solution takes about 1 hour to dry before you can paint over it.
Knotting solution is great stuff and a ‘must have’ if you are painting wood but what exactly is knotting compound? In this post, we provide the answers and give you tips on how and when to use knotting solution.
What is Knotting solution?
Knotting solution is a mixture of shellac and alcohol. It’s a thin liquid that is naturally brown in color and is used to finish bare wood before painting.
Also available is ‘White Knotting solution’ which is made using shellac that has been bleached to remove any color.
This can be useful when using light or pastel colors, as the bleached shellac does not stain the wood and will require fewer coats of paint to cover it over.
Shellac is a naturally formed resin that is produced by female lac bugs. These insects can be found in trees in India and Thailand.
The shellac is dried and sold as flakes, it is often mixed in alcohol to make liquid shellac, which can be used as a brush-on wood finish among other things.
Knotting solution is sold in DIY, hardware stores, and online in small pots, these usually come with a hand application brush fitted inside the lid,
Knotting is very sticky and it’s always a good idea to use disposable gloves and wipe down the pot to prevent the lid from becoming stuck.
Why Should You Use Knotting Solution?
Knots are found in all types of wood, particularly softwoods like pine or spruce. A knot is a circular patch of dense wood formed by the joint of a branch within the wood.
The surrounding wood grows around the knot to leave a grain pattern that flows around the knot, this gives the wood its beautiful pattern when cut, however, this can make the wood weaker as the joint is not as structurally strong as solid wood.
When knots are cut they can weep tree sap or resin for a long time and as this sticky liquid forms on the cut surface, it makes the surface sticky which will lift any paint and cause damage and discoloration.
This makes sealing any knots in your wooden workpiece an important step when preparing wood for finishing.
Using a Shellac based knotting compound will seal the surface and stop the knots from bleeding, which prevents discoloration and allows the surface to hold paint without damage
Knotting solution is intended for use on bare wood, applying a thin coating to the surface of wood helps prevent knots from weeping resin once they are painted.
How to apply Shellac Knotting solution?
When you are preparing the surface of your wooden parts, the surface is exposed by cutting and sanding, this removes any imperfections and turns a rough surface into a beautiful, smooth surface ready to paint.
Before adding the first primer coat the surface should be wiped clean with a damp cloth to remove any dust and debris.
Any excess resin should be cleaned away from the knot using a cloth and some white spirit, a heat gun can also be used to draw out any resin from deep inside the knot, just take care not to scorch or burn the surface,
Then give the knotting solution a quick shake, then using a small brush, apply a thin coat of shellac-based knotting solution applied to any knots on the surface,
Make sure to spread the liquid thinly and evenly over the area covering the whole of the knot, removing any drips.
Allow the knotting solution to dry for at least 15 mins before applying a second coat if necessary, several manufacturers recommend using two coats.
Check the manufacturer’s instructions to check the drying times before applying paint but I tend to leave the shellac to dry for at least an hour before painting
Once dry the surface should not be sanded or scraped as this will remove the knotting compound and allow the knot to bleed.
Then the surface can be painted using a good quality wood primer or varnish. Knotting solution is compatible with most wood primer and varnishes, but I recommend that you always check the labels or use a small area to test before you paint the whole surface.
It’s best to use knotting solution in warm temperatures between 12 – 25 degrees C and best not use it below 10 degrees or in high humidity as this will compromise the efficiency of the product.
Now let’s review each category in detail and consider the features, uses, and which drills we recommended for each section.
How to clean off Shellac Knotting compound
If you inadvertently spill or spread the knotting solution where it is not wanted, it can be cleaned up using a cloth with some methylated spirit before it dries too hard. Methylated spirit is a form of alcohol and will dissolve the shellac residue left when the knotting compound dries
To clean the knotting solution from brushes, leave them to soak for 12 hours in a glass jar with some methylated spirit, squeeze out any excess into a paper towel, and then wash the brush with soap and warm water.
To remove any old, dry knotting solution, cover the area in methylated spirit and use some wire wool to rub the surface clean, this may take a bit of time but will clean the surface of the wood very well.
As an alternative paint stripper can often be used but this is a harsh treatment and should be used sparingly.
How and where to store Knotting solution
As with most wood finishing products, Knotting solution should be kept in a sealed container in a warm, dry space.
I put keep on a shelf in a cupboard that I keep for wood finishes like oil paints and varnish, make sure the knotting solution is kept away from children,
Knotting solution is a flammable liquid and should be stored in a metal cabinet away from sources of ignition.
Knotting solution, if looked after and stored correctly at temperatures between 50 degrees F (10 degrees C) and 77 degreees F (25 degrees C), should last for at least 5 years as long as the container is fully sealed.
Find out more on how to treat wood for great long-lasting results in our in-depth post on painting Exterior woodwork
This article was written by: Richard Quinton – The DIY Help Desk Owner, Engineer & technical specialist.
Richard is one of the key partners in The DIY Help Desk team. He is a qualified Engineer, writer, and publisher, educated to Master’s level. He is a keen advocate of DIY and home improvements.
Richard enjoys helping others to learn new skills and reach their goals and believes that passing his knowledge and experience on through his writing is an effective way to positively impact the lifestyles and wellbeing of others on a larger scale.