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Old 08-17-2016, 10:35 AM   #1
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Default Thinned first coat question

I had a customer ask me a question I have never encountered before. It's common to cut or thin a clear coat as a first coat when finishing wood, but does it make any difference whether a full gloss or a duller finish is used to do it? I know it is common to use a duller finish for early coats to get a little more film build, but the purpose of the thinned coat isn't to provide film build. Does a duller finish have any advantages to using a gloss or semi-gloss to do this? Would it be any easier to sand maybe? Any ideas or comments?
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Old 08-17-2016, 05:07 PM   #2
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Originally Posted by PACman View Post
I had a customer ask me a question I have never encountered before. It's common to cut or thin a clear coat as a first coat when finishing wood, but does it make any difference whether a full gloss or a duller finish is used to do it? I know it is common to use a duller finish for early coats to get a little more film build, but the purpose of the thinned coat isn't to provide film build. Does a duller finish have any advantages to using a gloss or semi-gloss to do this? Would it be any easier to sand maybe? Any ideas or comments?
In the not-so-distant days of yore, it was, as you say, common to thin the first coat of a clear varnish (usually in a 3 coat system). You should always use a gloss for this, as your dull-rubs and satins would leave too much talc and stearates on the surface that could easily inhibit the performance, or adversely affect the sheen, of any subsequently applied coating. The purpose for this was to eliminate, or minimize, the effect of cissing when varnishes were applied, full-bodied, to a porous substrate. Thinning your first coat of varnish, was a more stable way to "seal" the surface as opposed to using the more modern varnish sanding sealers...

When poly-urethane varnishes made their impressive splash onto the marketplace, I'm sure you remember the warnings of using said Polys over stearate containing sealers, or shellac, and recommended cutting your poly gloss finish coat for a first coat application...second coat, apply full-body, and the third coat was for finish (either gloss, or non-gloss - e/g satin, dull-rub, hand-rub, flat, matte, whatever...) as non-gloss finishes were recommended as finish coats only. In comparison to gloss finishes, non-gloss films were far softer and did not provide a solid, hard, foundation for a 3 coat varnish system.

As alkyd varnishes have, unfortunately, almost become a thing of the past, and de-waxed shellac is now touted as a suitable seal coat for polys, many of these varnish sanding sealers are no longer available (and weren't that great of a product anyway)...and, due - in part - to the moving target of VOC compliancy, most manufacturers will not recommend thinning a first coat of varnish, or poly, with any solvent (other than acetone, which would be...stupid). I like your painter with these types of questions - I'm thinking he learned in an era when finishes were beautiful and didn't look so much like a melamine, or a laminated finish...sigh...
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Old 08-17-2016, 05:25 PM   #3
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In the not-so-distant days of yore, it was, as you say, common to thin the first coat of a clear varnish (usually in a 3 coat system). You should always use a gloss for this, as your dull-rubs and satins would leave too much talc and stearates on the surface that could easily inhibit the performance, or adversely affect the sheen, of any subsequently applied coating. The purpose for this was to eliminate, or minimize, the effect of cissing when varnishes were applied, full-bodied, to a porous substrate. Thinning your first coat of varnish, was a more stable way to "seal" the surface as opposed to using the more modern varnish sanding sealers...

When poly-urethane varnishes made their impressive splash onto the marketplace, I'm sure you remember the warnings of using said Polys over stearate containing sealers, or shellac, and recommended cutting your poly gloss finish coat for a first coat application...second coat, apply full-body, and the third coat was for finish (either gloss, or non-gloss - e/g satin, dull-rub, hand-rub, flat, matte, whatever...) as non-gloss finishes were recommended as finish coats only. In comparison to gloss finishes, non-gloss films were far softer and did not provide a solid, hard, foundation for a 3 coat varnish system.

As alkyd varnishes have, unfortunately, almost become a thing of the past, and de-waxed shellac is now touted as a suitable seal coat for polys, many of these varnish sanding sealers are no longer available (and weren't that great of a product anyway)...and, due - in part - to the moving target of VOC compliancy, most manufacturers will not recommend thinning a first coat of varnish, or poly, with any solvent (other than acetone, which would be...stupid). I like your painter with these types of questions - I'm thinking he learned in an era when finishes were beautiful and didn't look so much like a melamine, or a laminated finish...sigh...
I guess I should have mentioned that the question was about water based polys in general huh? Good answer though. But one question I have is aren't the stearates used to help the sand-ability of the finish? And since the first coat is to be thinned does it really effect the adhesion? You really can't use a product that is not stearate compatible over stearated products anyway, so why would the adhesion be an issue?
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Old 08-17-2016, 10:47 PM   #4
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Heres my non scientific additional .02. I would use a higher sheen for multiple coat clears when it comes to stains as it will not "muddy" graining in wood species. Final coat with a dull or medium rub sheen.

With lacquer we do not practice thin coating first coats but will often reduce the final coat up to 2:1 with retarder.

Only rationale to me to thin coat a first coat is to accellerate recoat time of an alkyd poly.
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