covering knotting solution outside

[sparkwright]

I have some knots on an outside window frame which cause a sticky goo to tear through the paintwork.

I think sunlight is causing this.

The knots have been knotted with solution - though I suspect the knotting solution may have been applied AFTER the primer.

***

I have scraped back the sticky mess to bare wood. I have removed the stickiness, cleaned it up, and let dry for a few hours. I have then applied aluminium wood primer, then undercoat, top coat.

The top coat is water-based.

The surface still bubbles up into a sticky mess.

***

Is it more likely this has been caused by, originally, knotting solution being put over primer?

I should stress it was later scraped back to bare wood before applying aluminium wood primer.

Would Zinseer BIN or something else solve this problem?

 

[blightymam]

Not sure if knotting really does anything when knots are in the hot sunshine.

Are there many large knots? Wondering if worth drilling them out and filling again, before painting. At least that way it would be the end of the problem forever!

Or of course, try BIN '-)

 

[JohnD]

For outdoor work, I am very fond of Aluminium Wood Primer, which not only seals knots and resin, but is very durable.

It dries grey, not silver.

No need to dab it, just use it to prime the whole thing. You can brush an extra (thin) coat over the knots first, if you want, to ensure good cover.

I suspect your knots are just very resinous. Black knots seem to be worst, but sometimes they can be persuaded to fall out.

If you use a hot air gun first, it draws the resin out of the knots and you can wipe it away. When cooled, it hardens and has to be sanded. Resin tends to soften and come out when heated by the sun, so black paint is especially bad.

Alcohol is said to clean off resin but I don't think I've tried it myself.

 

[sparkwright]

Is there a chance the knotting solution is what's bubbling up, rather than the knot itself?

Some of the bubbles seem just slightly to one side of the knots.

Although, as I said, I removed the sticky substance to bare wood (and then I used aluminium wood primer) and it still bubbled.

 

[misterhelpful]

It's the sap that bubbles and then escapes at the weakest point which is why it may only appear on one side. Removal of the knot is the best method to stop this, as some knots will bleed forever. You can heat them up and then wipe away excess with meths, but 'live' knots will soon bleed through again.

BIN will block the staining of sap from knots, but won't stop blistering.

 

[sparkwright]

Hmm. I could live with the staining, but the blistering is the main concern.

The blistering looks awful.

If I were to drill out the knots, how deep would I need to go?

Does the whole knot need removing, or just the surface?

How is the best way to drill out a knot? My idea would be a holesaw, then chip out the middle bit. Does this seem correct?

Is there a way of heating up the knots enough without having to use a hot air gun? I only ask this to try to avoid stripping too much paint away.

 

[misterhelpful]

A spade bit or auger bit is what I use, depending on the size of the knot. Obviously, if the knot goes right through the timber, you wouldn't want to drill all the way through to the inside. Just drilling around 1" deep should be enough in most cases and I would recommend using a wood plug to fill the hole where possible, or a two part filler.

 

[sparkwright]

Do I need to seal the hole with anything before applying the two part wood filler?

 

[misterhelpful]

Even when the knot is totally removed, I always applied some knotting just in case. There could still be some sap that has leached into the surrounding timber which could cause staining but, other than that, it's just a case of fill, paint, and enjoy a window that shouldn't bother you again. (At least not the knots!)