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woodstain v varnish - please explain the difference

Discussion in 'Decorating and Painting' started by jamiller, 8 Feb 2006.

  1. jamiller

    jamiller

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    Could someone explain the difference between the two products. Is coloured varnish essentially woodstain and producing the same results. I have a feeling that a lengthy but interesting answer may be on its way (Calling Canada) :D
     
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  3. Nestor_Kelebay

    Nestor_Kelebay

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    There are products called "wood stains" that are essentially similar to paint in that they hide the wood grain, but I have no knowledge of these products. I know about those wood stains that are intended to bring out the grain of wood.

    DIFFERENCE BETWEEN A DYE AND A PIGMENT
    A "dye" is a liquid that consists of molecules dissolved in a solvent (which could be water) that have color. A pigment is a SOLID particle that has color.

    INTERIOR WOOD STAINS
    An interior wood stain is nothing more than a dye dissolved in either white spirits or alcohol. Wood is not of uniform porosity, so when you paint that diluted dye onto the wood, more of it is absorbed into the more porous regions of the wood and less in the denser regions, and the result is that the softer wood becomes darker and the harder wood remains lighter. This is how wood stain brings out the grain of wood. After applying the stain, the white spirits or alcohol evaporate, leaving the dye behing INSIDE the wood cells. So, while paint and varnish effectively stay on the surface of the wood, stains penetrate right into the wood cells.

    EXTERIOR WOOD STAINS (and why the difference?)
    An exterior wood stain is different than an interior wood stain in that it will have some alkyd BINDER added to it. The binder is the component that forms a film in alkyd paints, but without any pigments in the can of stain, the binder will dry clear. The purpose of adding binder to exterior wood stains is primarily to coat the wood to prevent the rapid increases and decreases in it's moisture content due to large and rapid changes in humidity outdoors. It's these large changes in the wood's moisture content due to seasonal changes in humidity that cause wood left outdoors to crack. The binder added provides a film to house UV blockers to protect the wood from the Sun, and it also protects the wood from attack by mold, mildew and bacteria. Thus, the grey and cracked appearance of weathered wood is due to a combination of the effects of the changing moisture content in the wood from season to season and exposure to bacteria and the UV light from the Sun.

    WHY DOES WEATHERED WOOD CRACK?
    aside: (WHY does wood typically crack at the ends of any boards left outdoors. In a nutshell, the reason why is that wood absorbs moisture through it's end grain 15 times as quickly as it does across it's grain, and so the end cuts both become wetter faster and dry out faster due to any rain (and probably even humidity changes) the wood is exposed to. Also wood cells are like long narrow cylinders, typically about 80 times as long as they are in diameter. And, when wood absorbs moisture, only the cell WALLS become softer and thicker. Since there are 80 times as many cell walls per inch going across the grain than along the grain, the expansion of wood across the grain will be typically about 80 times as great. So, what happens is that if it rains, the ENDS of the board become wetter faster and so the ends of the board swell more then the middle. With subsequent sunny weather, the moisture is still being absorbed into the middle of the board, but the wood at the end cuts is drying out and shrinking. And so if the middle of the board is still trying to swell whereas the end of the board is trying to shrink, then the only way to keep both happy is for the wood to crack at it's ends.)


    The binder added to exterior stains, is an oil based binder (although you can get water based stains), and alkyd binders are impermeable to air and moisture, so they prevent that swelling and shrinking that causes the wood to crack.

    WHAT IS VARNISH?
    A true VARNISH is nothing more than a hardened (typically petrified) tree resin dissolved in a drying oil like linseed oil, safflower oil or Tung oil. The tree resins are called "copals". Amber is a very expensive copal, and to this day you can still buy true amber varnish for varnishing musical instruments like violins. (But for an expensive varnish like amber varnish, a less yellowing oil of the type artists would use to paint portraits (like poppyseed or walnut oil) would be used as the drying oil rather than linseed oil or Tung oil.)

    Since varnishes are meant to show off expensive hardwoods, they will be made both from oils that dry clearer and yellow less and will be made using a larger amount of more expensive copals that impart greater hardness to the film and discolor it the least. Consequently varnish will be made using a larger quantity of more expensive ingredients than an oil based paint will.

    WHAT IS AN "ENAMEL"?
    The word "enamel" only has a well defined meaning in the field of dentistry. Almost certainly the very first "enamel" paints were made when someone tinted a can of varnish in a paint tinting machine. Years ago, varnish only came in semi-gloss and gloss, and dried to a harder film than paint because of the larger quantity of copals dissolved in the oil. As a result, the "enamel paint" you got dried to a harder and smoother finish than a typical paint would, and to this day the word "enamel" is meant to suggest a paint that dries to a harder smoother film than you'd otherwise expect. However, because of ongoing improvements in the plastics used to make paints, EVERY paint dries to a harder film than it did 20 years ago, so some paint companies have come to slapping that word "enamel" on every can of paint they make save their dead flat emulsion paints. In that case, the word "enamel" is nothing more than a racing stripe designed to convince you to buy their paint instead of the other company's paint (which is equal in quality).

    WHAT IS POLYURETHANE?
    Nowadays, polyurethane has replaced varnish as the clear coating of choice over wood because it costs less and dries to a clearer, harder and more protective film. Essentially, polyurethanes are best thought of as alkyd resins on steroids. They make an alkyd based polyurethane by adding something called an "isocyanate" which is anything with a (-N=C=O) group in it to the pot when making alkyd resins. The isocyanate groups react with the hydroxyl (-OH) groups in the glycerine to form "urethane" groups (-NH-(C=O)-O-) right inside the alkyd resin. Urethane groups are very strong (and I don't know why), and they very much act like a roll cage inside a race car, making the alkyd resin harder (if you could squeeze it) and stronger (if you could stretch it). As a result, polyurethane "varnishes" dry to a harder film than a true oil based varnish.

    COLORED VARNISH STAINS
    A colored stain is nothing more than an exterior wood stain to which pigments have been added to impart color to the binder film which remains on the surface. So, when you apply a colored stain, the first thing that happens is that the dye dissolved in the white spirits or alcohol is absorbed into the wood (preferentially into the more porous regions) thereby bringing out the grain of the wood. The binder resins and colored pigments are too small to be absorbed into the wood, and remain on the surface of the wood to provide a colored protective film on top of the wood. And, depending on how much pigment you add, that film can be anything from nearly clear to opaque.

    Anyhow, that's my understanding of stains and colored varnishes, and that understanding is correct so far as interior and exterior stains, varnishes and polyurethane goes. I really don't know for sure if an opaque exterior "stain" is really just an exterior stain with so much pigment in it that the binder film that remains on the surface is both colored and opaque. In that case, the only difference then between an opaque exterior stain and a paint is that the stain would have dye in it that is absorbed into the wood to bring out the grain. But putting dye into an opaque exterior stain in my view stands right next to putting in a hearing aid and turning it off. The opaque binder would hide the wood grain anyhow.
     
  4. 2scoops0406

    2scoops0406

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    Played for and got!
     
  5. Nestor_Kelebay

    Nestor_Kelebay

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    I think we all agree that it's long. I'll let you guys decide if it's interesting.
     
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  7. Zampa

    Zampa

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    :LOL: :LOL:
     
  8. 2scoops0406

    2scoops0406

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    I'm just poking fun, English sense of humour, please keep posting :LOL:
     
  9. Zampa

    Zampa

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    Nester...could you repeat that please!... ;)
     
  10. jamiller

    jamiller

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    Thanks Nestor - as previous posts suggest, just English sense of humour. This board would be a poorer one without your input so keep em coming!! ;)
     
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