What would be an appropriate clear finish for a bathroom sink cabinet? I’m just thinking about the water/moisture issue. I’d like to use poly but maybe I should use spar varnish instead? I don’t spray–at least not yet. On interior applications, is polyurethane as water resistant as paint?
Brian
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Spar varnish isn't necessary, it's not more moisture resistant than other oil based varnishes, it is more flexible and more UV resistant. Similarly, polyurethane varnish doesn't have much of particular value for bath cabinets--it's major benefit is resisting heavy abrasion.
My recommendation for finishing schedule on bath vanities is first dye and/or stain as desired. Then seal with dewaxed shellac--a coat or two of 1 1/2 lb. to 2 lb. cut--provides substantial resistance to water vapor transfer into the wood. But the shellac then needs covering with a durable top coat--which without spraying pretty much comes down to oil based varnish. Wipe on is generally the easier to apply. Waterlox is good albeit a bit on the dark side, and Pratt & Lambert 38 or Cabot Varnish (not polyurethane) for lighter varnishes. Brushing varnish takes fewer coats, but likely more rubbing out.
I appreciate your input, and I understand that you don't like poly, but am curious if any of the varnishes you recommend are more water resistant than poly (if so, do they come in gel form?). Also I don't understand how shellac under a film finish can improve water resistance. Do varnishes breath more than shellac or something? Is paint a more impervious barrier than varnishes (just for the sake of comparrison)? Sorry to be difficult, but I'm just not looking for an advanced finish right now. I will get there someday. Take care.Brian
Consumer grade single part polyurethane varnishes (poly) and traditional resin varnishes differ insignificantly among them with respect to moisture resistance. Within the context of protecting bathroom vanities any difference among the various varnishes shouldn't matter. The consumer poly is not pure polyurethane resin, mostly it is alkyd resin modified with added polyurethane resin--it's not a whole different class of finishes, just a modification whose main positive attribute is extra wear resistance which is of particular value for floors. The addition of the polyurethane resin brings some negative attributes, too. It is more difficult to rub out, and is more picky as far as what substrates to which it will adhere. In heavy coatings it can be subtly less clear and changes the way light is refracted within the finish. (At least I think that's why I am aware of the difference between poly varnish and traditional resin varnish more at some angles than at others.)
It does turn out that shellac is more effective than varnish at slowing the transfer of water vapor into and out of wood. There are USDA Forest Products Laboratory studies that demonstrate this. So if protected from the household chemicals and liquid water by the varnish on top the combination is more protective than each alone.
Paint has durability advantages over varnish when UV light is part of the equation. The pigment in the paint block light from reaching the wood where it could cause a breakdown of the surface fibers that could cause the adhesion of the finish to to the wood to fail. Otherwise, there are so many differences among paint types I don't know how to generalize further.
Gel varnish should perform about the same as standard wiping varnish. I say should be because in my fairly limited experience with gel varnish I have been a bit disappointed. It is easy to use, and generally satin finish if that's an advantage. As a wiping varnish it takes more than one or two coats to achieve a fully protective film.
Edited 4/1/2008 6:22 am ET by SteveSchoene
Wow, you know a lot about finishing. Glad I was able to get you going. Thanks for answering my questions. Talk to you later, Steve.Brian
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