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persistent knots

Discussion in 'Decorating and Painting' started by sparkwright, 4 Oct 2015.

  1. sparkwright

    sparkwright

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    How can I prevent persistent knots bleeding through the paintwork?

    This is on a new wooden window frame. This is outside, and has been exposed to all kinds of weather including lots of sunshine in the space of one year.

    Pre-primed wood, knots were already just showing a brown mark through white primer. Water-based top coats used.

    The paint just bubbled up quite a lot where the knots are.

    Have twice tried to rectify this.

    First time, scraped back blistered paint, removed paint where knot was. Cleaned with white spirit. Knotting solution, oil-based primer, oil-based undercoat, water-based top coat. Blistered up.

    Second time, like above, except used Dulux aluminium wood primer instead of the knotting solution and regular primer. On top of the aluminium wood primer I used an oil-based undercoat and then water based top coat. Blistered up again.

    I'm using water-based top coat because it matches the rest of the building.

    The top coat is mostly dark green. There is some white top coat too, but the knot blistering isn't as bad on this, though still slightly occuring.
     
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  3. Robbie uk

    Robbie uk

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    Did you use a water based knotting or the shellac based one ?
     
  4. stevethejoiner

    stevethejoiner

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    I don't know if this will help, but we recently refurbished some old windows where knots were a problem on the old paint finish. When we were stripping back with a hot air gun to remove all paint, we experienced the sap bubbling from the knots as we passed the hot air gun over them. In fact the sap that was coming out was so hot it felt like boiling tar, and these were 50 year old windows that were south facing, so had many years of hot sun on them, I was surprised sap would still be there. Anyway, after cleaning off and applying knotting, oil primer, undercoat and gloss the knots did not show through again. So I wonder if you applied heat over the knots you could draw out the sap?
     
  5. joe-90

    joe-90

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    It'll come back for ever. The best bet is to drill out the resinous knot half and inch and fill with a polyester paste such as Fibral. Then it won't come back.
     
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  7. Robbie uk

    Robbie uk

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    Joe is probably right. I painted the outside of a house every five years for 25 years and the knots always bled as soon as the sun came out. Didn't matter what we put on them.
     
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  8. sparkwright

    sparkwright

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    Half and inch?

    Is that one and a half inches?
     
  9. joe-90

    joe-90

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    No, just enough to seal will do.
     
  10. sparkwright

    sparkwright

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    Out of interest, is this something to do with the wood itself? Is it the 'wrong' type of tree, or the wood doesn't get seasoned for long, or do dark paint colours attract the heat more??
     
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