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Old 02-21-2009, 01:44 PM   #1
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Best primer for preventing wood knots from bleeding through?


Right now I'm using Bulls Eye water-based primer/sealer on fairly clear pieces of poplar.

I will be using some pine boards that are pretty clear, but still have some knots.

What's the ideal primer that will prevent bleed through?

Thanks in advance

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Old 02-21-2009, 04:03 PM   #2
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Best primer for preventing wood knots from bleeding through?


use shellac

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Old 02-21-2009, 04:37 PM   #3
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Best primer for preventing wood knots from bleeding through?


Thanks Bob.

I really try and stay away from oil-based products - is shellac oil-based?

Is there no water-based primer that would do the trick?

Can I use Latex paint over that?

Thanks again.
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Old 02-21-2009, 05:58 PM   #4
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Best primer for preventing wood knots from bleeding through?


If you want to paint over knots, shellac is the only thing that I know of that will seal up the knots. When I refinished by upstairs, I used seconds lumber for all of my door and window jams. Get a two dollar brush at home depot and prime the wood with two coats of shellac, when you are done just throw the brush away. It does not look like clear lumber, but is pretty close.
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Old 02-21-2009, 09:07 PM   #5
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Best primer for preventing wood knots from bleeding through?


Most all the shellacs that you can purchase at a store will be oil based. Shellac is the residue of an insect in some far off place. It is refined to remove impurities but then needs to be thinned by way of a petro product to allow it to be spread and set on to the wood surface. I'm not sure of any non tox way to apply it but I'm pretty sure that shellac is a very safe product after it cures, in fact I think most childrens toys are shellacked and that it is even FDA approved for food contact. Fruit has shellac applied to it to help preserve it. Maybe some others could give more expertise.
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Old 02-21-2009, 09:39 PM   #6
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Best primer for preventing wood knots from bleeding through?


i think shellac is alcohol based
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Old 02-21-2009, 10:29 PM   #7
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Best primer for preventing wood knots from bleeding through?


Whenever something bleeds through a paint or primer, it's because it's soluble in the water or mineral spirits base of the primer. Ball point pen ink will bleed through oil based primers cuz the ink is soluble in the paint thinner of the primer.

Wood has "tannin" or tannic acid in it, and some woods have more than others. Southern yellow pine has a particularily lot of tannin in it, especially in the knots. Tannin is soluble in water, so it will bleed through latex primers like Bullseye 123 as the tannin in the wood dissolves in the water of the primer and discolours it.

The best way to deal with bleed through problems is by using a different primer. Very few things are soluble in both water and mineral spirits. So, if the tannin is bleeding through a latex primer, then switch to an oil based primer.

Another option would be to switch to a shellac based primer like BIN. Tannin isn't soluble in either mineral spirits or alcohol. (Shellac based primers are alcohol based cuz shellac dissolves in alcohol.)

The other thing to keep in mind is that some primers, notably Bullseye 123 have VERY POOR HIDE. What you may think is bleed through may in fact be really lousy hide.

PS: and you don't need to know this:
Technically, a "lacquer" is anything that forms a film by evaporation of the thinner alone. There can be no chemical reaction (as there is in oil based paints) or coalescence (as in latex paints) after the thinner evaporates to be considered a "lacquer". Shellac is one of the few true lacquers we still use. As the alcohol evaporates, the shellac that was dissolved in it remains behind to form a film. And, similarily, you can re-dissolve that dry shellac with alcohol too.

You can use either an oil based primer or a "white pigmented shellac" like BIN to block that tannin bleed through.
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Old 02-22-2009, 11:03 AM   #8
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Best primer for preventing wood knots from bleeding through?


Oil based does a bad job sealing knots, though not as bad as water based. Shellac is the only primer I've ever seen work. I've heard some people recommend brushing a coat of carpenter's wood glue over the knots as a primer and report that it works just as well as shellac. I haven't tried that to confirm it though.
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Old 02-24-2009, 10:44 AM   #9
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Best primer for preventing wood knots from bleeding through?


Either inside or outside, any bare wood should be primed with oil. Like Nestor mentioned, the Tannins in the wood will bleed right through latex primer. Latex primers will also lift the grain of the wood and a lot of times the grain will not go back down when it is dry. If I can have access to the wood before installation then that is better. I will take my trusty dusty can of spray bin and hit any knots in the wood. Then I will prime the board with oil.
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Old 02-25-2009, 08:06 AM   #10
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Best primer for preventing wood knots from bleeding through?


I agree with Nestor and Mathew. You can however buy a waterbourne shellac but it does not hold back the bleed of knots. I always use oil based primer to prevent acid bleed on cedar claps shingles but only alchohol based shellac works on knots. Not to be nasty, but some people should know about a topic before advising people.
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Old 02-25-2009, 08:53 AM   #11
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Best primer for preventing wood knots from bleeding through?


Quote:
Originally Posted by waynech View Post
I agree with Nestor and Mathew. You can however buy a waterbourne shellac but it does not hold back the bleed of knots. I always use oil based primer to prevent acid bleed on cedar claps shingles but only alchohol based shellac works on knots. Not to be nasty, but some people should know about a topic before advising people.

well you didnt say anything anybody else didnt say you didnt need to add your last sentance
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Old 02-25-2009, 09:34 AM   #12
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Best primer for preventing wood knots from bleeding through?


Quote:
Originally Posted by waynech View Post
I agree with Nestor and Mathew. You can however buy a waterbourne shellac but it does not hold back the bleed of knots. I always use oil based primer to prevent acid bleed on cedar claps shingles but only alchohol based shellac works on knots. Not to be nasty, but some people should know about a topic before advising people.
I would imagine it is not truly a shellac like we are used to. You would have to cut the alcohol with water prior to mixing which would limit the amount of actual shellac in the product. I would like to see how much shellac is actually in the waterborn shellac as true shellac uses 3 lbs per gallon.
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Old 02-25-2009, 10:41 AM   #13
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Best primer for preventing wood knots from bleeding through?


What you are missing here is the permeability of the different shellacs. You want to decrease tannin transfer. You need the oil based.
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Old 02-25-2009, 05:13 PM   #14
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Best primer for preventing wood knots from bleeding through?


Spot prime with a shellac/alcohol-based sealer

I've had latex/acrylic/waterborne work for a few days, even weeks (though even over-night is rare)
Even if it is an other-wise quality product labeled for sealing knots
Most of the time it bleeds through pretty quick
Not good enough for me to recommend them for sealing knots by any means

Oils last longer (before bleed-through) and may often be just fine
I have even had them bleed through though
(though not through two coats oil/alkyd as I recall)
I can recommend them, but with the caveat it may need two coats

Best to be sure, and spot the knots with a shellac/alcohol-based product
-admittedly depending upon product and/or actual knots it's (slightly) possible this also may need two coats, but at least you'll know right away (it'll bleed through rather fast-before you continue to the next step as opposed to after you've finished the project) and you are only dabbing the non-water-based product in a few spots rather than coating the whole thing
And when you're done spot-priming, that's it for the solvent-based products
After spotting the knots, you can then prime the whole thing with latex/acrylic
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Old 02-26-2009, 07:49 AM   #15
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Best primer for preventing wood knots from bleeding through?


Get a pint of Bin primer which is an alochol based primer/sealer. Not to beat a dead horse but i'm pretty much repeating what most said before. Bin is basically white Shellac. If you have clear shellac - you can use that too. But just use Shellac - which is alcohol based.

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