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  1. #1
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    Default Painting to cover knots in wood

    I have tried to paint over knots in wood. I have already applied at least two (2) coats of white high gloss paint only to find that the knots bleed through.

    Any thoughts?? Thanks

  2. #2
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    Default Re: Painting to cover knots in wood

    to seal the knots you need to seal them with a shellac based primer. zinsner makes it. get the can with the yellow label its both alkyd and shellac based
    fire up the saw and make some dust

  3. #3
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    Default Re: Painting to cover knots in wood

    Thanks for the advice.

    I will let you know how the project went. I have several knots to cover. What kind of paint paint would you suggest after the knots have been prepared for painting?

  4. #4
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    Default Re: Painting to cover knots in wood

    The primer/sealer jkirk was probably referring to is "BIN" sealer, which is a white shellac made by the Zinsser company, and available from Home Depot (and competitors).



    You need to understand that whenever a stain "bleeds through" a primer or paint film, what's actually happening is that the stain is:

    1. dissolving in the primer or paint you're using, and

    2. diffusing through the wet primer or paint film to discolour the surface of that film so that when the primer or paint dries, there is a discolouration on it's surface.

    This is how ALL stains "bleed through" primers and paints.

    In your case, what's actually happening is that the tannins in the knots of the wood are dissolving in the water based primers and paints you're using.

    To prevent that from happening, you can use an oil based primer (since tannins aren't soluble in mineral spirits), or you can use a shellac based stain sealer (cuz tannins aren't soluble in alcohol either).

    Once you use an alkyd primer or a shellac based primer over your knots, you can use any paint you wanna over your wood. However, the best choice of paint to use depends entirely on what type of service that wood is gonna see. You can use a latex paint on a wood molding (like crown molding) that's gonna spend it's life without seeing any hard wear and tear. But, if this wood is on a table top, shelf, mantle or is meant to form any other working surface, then you'd better use a much harder drying paint, like an interior alkyd or an alkyd based polyurethane paint. There are very much harder drying paints available, such as moisture cure polyurethanes and epoxies, but those are normally used over metals rather than wood, cuz in the case of epoxy over wood, it'd be the wood that would be the weakest link in the chain.

    So, you can simply prime or paint over your knots (or the ghosts of your knots) with any primer the tannins won't dissolve in, such as an alkyd primer or shellac based primer. Then paint over everything with an appropriate paint based on the service this wood will be put to.

    PS: You don't need to know the rest...

    There is no good definition of the word "tannin". The word got it's name because extracts from trees were used in the leather "TANNING" industry throughout the 1800's until better chemicals came along. Suffice it to say that all tannins are polyphenols.

    A phenol is a benzene ring with at least one hydroxyl group, like this:



    It's that hydroxyl (-OH) group that makes phenols soluble in water.

    Gallic acid is a good example of a phenol:



    And, in fact, if you stick 4 gallic acids around the periphery of a central glucose molecule, you get a POLY-phenol called "tannic acid":



    Tannic acid is just one of the polyphenols that's dissolving in the latex primers and paints you're using which is causing you all the grief with "bleed through".

    In fact, it's estimated that up to 40 percent of the mass of a tree's leaves consist entirely of polyphenols. And, collectively, all those polyphenols in wood are known as "tannins". Tannic acid is just one of very many different tannins in a tree.

    It's all those hydroxyl groups sticking out all over the place that make the tannins in wood soluble in water. So, if you prime or paint over certain species of wood (notably cedar, Southern Yellow Pine, Redwood and red oak) with a water based product, you'll get discolouration of the primer or paint because of the tannins dissolving into and diffusing through the primer or paint.

    The solution is to use an oil or alcohol based primer over the wood (or just over the knots) which the tannins won't dissolve in.

  5. #5
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    Default Re: Painting to cover knots in wood

    the can shown in nestors post is by the right company but not the rigt type. get the yellow labelled can
    fire up the saw and make some dust

  6. #6
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    Default Re: Painting to cover knots in wood

    Quote Originally Posted by jkirk View Post
    to seal the knots you need to seal them with a shellac based primer. zinsser makes it. get the can with the yellow label its both alkyd and shellac based
    I'm not aware of any coating made by anyone that uses both alkyd resins and shellac in one product.

    Could you please provide a link to this Zinsser product so that I might take a closer look at it.

    (I really don't see how such a product could work. Alkyd resins crosslink by reacting with the oxygen in the air, whereas shellac dries to a film by simple evaporation of the alcohol it's dissolved in. In my view, the shellac would interfere with the film formation mechanism of the alkyd resins. Shellac molecules would get between alkyd resins and prevent them from crosslinking with each other. The result would be a partially cured soft film that would probably be inferior to either a shellac or alkyd film. I see absolutely no sense in mixing shellac and alkyd resins, and I'd like to see what Zinsser is trying to do here. Please provide a link or more info cuz I can't find this product o n l i n e anywhere.)

  7. #7
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    Default Re: Painting to cover knots in wood

    like i have said twice the yellow labelled can of bin by zinnsers. i have two cans of it right now which i used for a job last week. some stores may not carry it in your area. which is highly possible. some carpenters will use shellac alone first to seal the knots and then use bin as a second layer

    i work in a coastal environment doing a large amount of high end exterior finish work. normally we use a similar product which we buy directly from the mill who makes our exterior wood trim but if they are out we use the alkyd based bin to seal pine

    anyhow, heres teh link
    http://www.homedepot.ca/product/zinn...r-946ml/943741
    fire up the saw and make some dust

  8. #8
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    Default Re: Painting to cover knots in wood

    Any good stain blocking primer will work, though you'll get your best results with either a shellac or lacquer type sealer. I always kept a spray can of clear lacquer in my truck for ink stains, knots, etc., that I would come across in my painting duties.

    Another tip when sealing knots. You're going to get "shiners" wherever you apply the sealer, as these areas will change the characteristics of the top coat. In other words, sealers tend to have a gloss to them, when top coated with a flat, matte, or eggshell type paint, the sealed spots are going to be noticed. Because you're using high gloss paint, there shouldn't too much to worry about.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  9. #9
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    Default Re: Painting to cover knots in wood

    I was able to download the MSDS and TDS for Zinsser's Cover Stain from here:

    http://www.rustoleum.com/cbgresourcecenter.asp

    It calls itself an oil based primer and there's nothing in either the MSDS or TDS to indicate there's any shellac in it.

    The MSDS says that there's both mineral spirits and petroleum distillates in it, but it doesn't mention any alcohol at all.
    The TDS says to clean it up with mineral spirits or paint thinner.

    I don't believe there is any shellac in it. I think it's much the same thing as KILZ cuz the MSDS does say it contains naptha, but it doesn't say how much.

  10. #10
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    Default Re: Painting to cover knots in wood

    you read way too into this stuff nestor, very few proffesional contractors would go the extents that you do to find out the amount of pointless info you are going for other than what works or doesn't. let it rest.

    the bin i recommended is used by many high end painters whom i know, hence why i use it they only use what works. shellac is a natural wax by the way which comes from trees. just the newer versions have been chemically modifed to make it easier to use
    Last edited by jkirk; 07-31-2011 at 09:06 PM.
    fire up the saw and make some dust

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