How to stop the blisters forming over the knots?

Questions & AnswersCategory: Exterior Trim, Windows and DoorsHow to stop the blisters forming over the knots?
Anonymous Staff asked 2 years ago

Cedar releases natural oils, sap, and tannin. This release occurs at the knots. Blisters are forming over each knot. Scraping, sanding and priming may fix a few but then more blisters come up in different spots, and sure enough, the new spots have knots too.. Am I chasing this around like a chicken with no head……How can this blistering be stopped? I have used BIN Cover Stain on it. The BIN work great everywhere else but the knots…… A dark color is being used, oil based, applied in hot weather using Penetrol to slow drying and increase leveling. The blisters/bubbles go down to the knot, not inner layer paint bubbles..No problems with paint adhering to primer.

Is this a lumber problem, or a painting process problem? Virginia Cedar….A deep red cedar for sure, high aroma and red with little white grain…….

Any suggestions on how to stop the blisters forming over the knots?

1 Answers
crowderpainting Staff answered 7 years ago

This is a combination of new fresh cedar and a dark color. The dark color is absorbing heat from the sun and this helps to draw out the sap from the wood. It could take years to truly dry-out and stop most of the sap.

You will have to fix the blisters when ever they happen. Instead of using Cover Stain use a shellac primer on the knots. Make sure to apply 2 coats of the shellac primer with some dry time between each coat, usually an hour or less depending on the temperatures and humidity.

The shellac primer should stick better than an oil based primer but this depends on the amount of sap remaining in the wood.

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