How to stop Knot Bleed – a Full Guide

Updated Jun 12, 2024 | Posted Jan 16, 2024 | Professional insight, Product Advice | 0 comments

Sap, which carries on seeping from timber long after it’s been harvested, tends to bleed through any paint finish unless treated. In this blog, I’m going to explain how to stop knot bleed on timber. I’ll talk through both bare and previously painted wood. I hope it helps.

 

How to Stop Knot Bleed on New Wood

 

If you intend to varnish or stain your wood, then you do not need to take any measures to stop knot bleed. Varnish and stain tends to hold it back anyway. If you intend to paint the wood, then you do need to take action.

The traditional way to stop knot bleed when painting new timber is to prime the knots using a product called white knotting solution (Click here to see prices). This is a very thin shellac that soaks into the wood and prevents sap from seeping.

You can use white knotting either before or after your first coat of wood primer. Just ensure you don’t miss any, otherwise you will still experience problems further down the line. You can apply white knotting with a cheap paintbrush or a lint free cloth. (I tend to use a cloth when I only have a little bit of wood to paint, but it’s far easier to apply the solution with a brush when working on more timber).

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What is the Best Primer to Stop Knot Bleed on Bare Timber?

The science of paint has progressed rapidly in recent years, and there are now several products you can use to prime your timber that will also block knot bleed. This saves time and money. Valspar Trade Knot Block is a good example, as is Tikkurila Multistop, but my favourite is Zinsser BIN Aqua.

This is a fully water-based adhesion primer, stain block and knot blocker in one. It has great opacity (which often means you don’t need to apply as many topcoats). It’s easy to use too, and creates the perfect base for additional coats of paint. Zinsser BIN Aqua is a product I can wholeheartedly recommend.

Zinsser BIN Aqua – a water-based adhesion primer, primer, stain block, and knot block in one.

How do You Stop Knot Bleed on Previously Painted Timber?

 

Knot bleed on previously painted timber is unsightly. As a Decorator, I must have fixed hundreds of front doors over the years that were ruined by knot bleed. It takes a slightly different approach to bare timber.

I can remember my old boss telling me we had to burn off the existing paint, prep the wood, prime the knots, then repaint. However, I have slightly better news for you; you don’t need to go to those lengths to stop knot bleed on previously painted wood.

The easiest way to deal with the issue on interior wood is to use Zinsser BIN (red tin) as an undercoat.

This is a shellac-based product and is easily capable of stopping knot bleed in its tracks. Use a cheap paintbrush or foam roller to apply this paint as you will need to throw them away when you’ve finished.

Shake the tin too, as the solids in Zinsser BIN tend to settle on the bottom of the tin. Other than that, you’re good to go.

A stain block to stop mould from bleeding through emulsion.

If you’re dealing with knot bleed on exterior timber, then you should go for Zinsser BIN Aqua rather than Zinsser BIN. This is because the shellac version is brittle over large areas, so it won’t cope with natural movement of timber when outside. (Click here for online prices)

 

Final Thoughts

 

There you have it. Knot bleed is horrendous, but there’s always a quick fix. Knotting solution is the traditional method when painting new timber. Zinsser BIN Aqua can be used on bare or previously painted timber. Zinsser BIN (shellac) is great for previously painted timber inside.

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FAQs

 

What is knot bleeding?

Wood knots are basically where branches grew from trees, and the bleed is from sap still contained within the knot. If unsealed, the sap will bleed through your paint finish and ruin its appearance.

 

Can you paint over knotting?

Yes, knotting solution is designed to seal knots before painting. So, painting over knotting solution is no problem at all.

 

Does undercoat stop knots?

No. There are a few primers on the market such as Bedec All Prime and Zinsser Aqua BIN that you can use as an undercoat and will stop knot bleed. However, most other undercoat products will not seal knots.

 

Do you sand after knotting?

Yes. Usually you would seal the knots, prime, then sand your timber. If excessive sanding is required, then you might play it safe and apply another coat of knotting solution, but this is rarely necessary.

 

Will Zinsser Cover Stain cover knots?

Zinsser Cover Stain is not recommended for sealing knots. However, you can use Zinsser BIN and Zinsser BIN Aqua.

Updated Jun 12, 2024 | Posted Jan 16, 2024 | 0 comments

About the Author

About the Author

Mike Gregory is a Professional Painter and Decorator who works in the Northwest of England. He mainly sub-contracts for large decorating firms and works on a wide variety of projects.

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