Dealing with knots "bleeding" through paint

Question

I'm sure that people have seen this where you would be painting something white or lightish colour only to have the resin from the knots bleed through paint sometime later. Similar to what you see in the following picture.

For my latest project I knew this was a possibility so I coated the knots with some shellac. However after about a month it still came through. I am sure if it was dried properly this might not have happened but the project was a large storage shelf that did not fit in my oven.

So I have 2 related questions:

1. What should I have done to try and prevent this?
2. What can I do to correct the damage after it has occurred.

Answer 1

What should I have done to try and prevent this?

Instead of shellac-based primer as already recommended (although very effective it can be inordinately expensive) I was actually going to recommend shellac itself, but then I read you'd already tried this and it wasn't effective.

However, I think the reason you got bleed-through wasn't a fault in the shellac but probably just that it wasn't applied in a heavy enough 'cut' (cut traditionally refers to dilution rates for shellac when made up from scratch). Rather than make a heavier cut you can simply apply further coats since it's the total thickness of the shellac layer that is important, not how many applications it took to achieve it.

Unfortunately I can't recommend a specific number of coats. Erring on the side of caution I suppose three would be the minimum to apply, and as one of the chief advantages of shellac is the very fast drying time this shouldn't take that long. Apply more than three if patience allows.

What can I do to correct the damage after it has occurred.

The bad news you were probably expecting: you should really remove the paint, treat the knots and re-paint.

Answer 2

Shellac primer is the way to go maybe more than one coat and you should allow it to cure, which is different than drying.

I use spray cans of this: http://www.homedepot.com/p/Zinsser-1-qt-B-I-N-White-Advanced-Synthetic-Shellac-Primer-4-Pack-271009/205421238?cm_mmc=Shopping%7cBase&gclid=CJP986HPzsYCFQ-PaQodAm0B7g&gclsrc=aw.ds

Spray the spots that are bleeding, lightly sand them and then apply an even coat of paint, ideally to the whole wall, though you can do the spot and feather the paint out from there...the potential drawback is you may get flashing on the paint if you don't do the whole wall.

If the first coat of primer still bleeds through, sand lightly to smooth things out and spray on another coat.

Any oil based, or shellac based primer should do the trick, though it may take more than a single coat. I know we don't do product recommendations...but I am recommending that one as I have used it many times in the last 15 years and it works great.

Oh...and shellac based primer...really really stinks.

Answer 3

Correcting after it has occurred: only solution I can think of is to re-seal (shellac's single nicest feature may be that it sticks to both water- and oil-based finishes) and repaint (might be able to get away with just painting that area, feathering out the edges of the new paint to blend it with the surroundings).

Previous owners of my place solved this by going the other way -- they used white stain on the porch, so the wood shows through a bit everywhere and any bleeding knots just add a bit more rustic character.

Answer 4

Has no one else of thought of using knotting solution? It's an oil based coating for bare wood that you paint on the knots. This stops the bleed through to the paint. My house is full of white painted pine. Works a charm.

Answer 5

The only method I have found that works 100% to hide these stains is to use a two part epoxy marine paint as your primer. Reason being that two part epoxies cure by chemical cross-linking as opposed to most paints that cure by solvent/water evaporation. The cured epoxy film makes an impervious barrier for the wood resins. Of course these epoxy paints are expensive so the moral of the story is: don't use knotty pine if you can afford clear pine ( or whatever species suits your fancy).

Answer 6

There are 3 ways to stop a knot bleeding through:

1. Remove the knot and either plug it with timber or 2-pack wood filler
2. Heat treat it, effectively drawing all the tannins from the knot like a summer would do over many years. Then apply J H Ratcliffes Stytic Sealer with two coats, then finish in normal way
3. Don't use timber with knots in.

Answer 7

My two cents...

Remember, when working with shellac, you must use a "fresh" batch. Nearly all shellac is only viable for about a year after flakes have been mixed with alcohol. Premixed brands claim a three year shelf life, but in my opinion that is a stretch.

Shellac should be all that is needed to block nearly any bleeding.

Answer 8

More work and possibly not feasible but would it solve the problem if one drills out the knots and replaces with a dowel? Thinking of grain direction too.

Answer 9

I have used ordinary wood filler, just like you would fill a hole or indentations.

This makes the paint color fine and consistent, but there will still be a round flat spot if you look closely, so it may be impossible to remove all signs of the knot.

Answer 10

primer I have tried everything, I found rustolium grey aerosol primer sealer or krylon aerosol spay primer. These seem to stop the bleed through recoat with kilz primer (premium) then top coat with the color. These seal the wood from the paint. Just make sure the can states it is suitable for wood