I want to paint a knotty pine panel and saw a recommendation to use two-part epoxy marine paint as primer so the knots would not bleed though the paint as few years later. Do I use the marine primer and then paint finish product or do I just apply few coats of marine paint and no primer product?

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If all you're aiming for is preventing knots from bleeding resin through your paint you don't need to resort to using an epoxy paint, just shellac will do the job perfectly well. Shellac has been relied upon for this for many many years and it works as well today as it did in the 19th century.

You can either treat the knots individually* with shellac (one coat of strong shellac, e.g. 4lb cut, or a few coats of weaker shellac, e.g. 2lb cut) or because it's now available an alternative would be to prime using a shellac-based primer. These are specifically made and marketed as stain blockers and they can work very well. However, they can be quite pricey.

Plain shellac is relatively inexpensive, especially if made up from dry shellac flakes or buttons dissolved in denatured alcohol, rather than bought ready-made in a tin. It also lasts much longer in dry form than once made up into solution.

*Guides to using shellac for this purpose disagree about when to apply it. Some say it should be applied to the bare wood prior to priming and others that it should be painted on the knots on top of the first primer coat. I can't say which is more reliable, but because there is much published advice to do it in both ways my suspicion is that it works equally well either way.

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