Thinning Finishes

[javajake]

I have some finishes such as Bullseye shellac and Cabot spar varnish that say "DO NOT THIN" on the label. Is there a good reason for this? Should I ignore it, or would that be a bad idea?

[wingarob]

I've never paid any attention to those instructions .. and have also never been displeased with the results. I don't know why they do that, maybe some fear that their product might not perform properly if thinned beyond a certain point. When I use shellac, it's usually 1/2 - 1# cut. Varnish, I normally thing about 75% VM&P Naptha - 25% varnish. I've used lots of brands of varnish/poly and have always had good results doing it that way.

[thewoodwhisperer]

Yeah I believe its a VOC thing as well. Although some finishes will not behave properly if thinned to far. But even then, "DO NOT THIN" is a little aggressive. Maybe, "Please do not thin beyond x%" or something like that. I have thinned things like shellac, lacquer, and oil-based varnish substantially with no major detrimental effect. Now water-based finishes, those are another story. You really don't want to thin them too far since water isn't actually the thinner. So you can ruin the finish by thinning with too much water.

[javajake]

Yeah I believe its a VOC thing as well. Although some finishes will not behave properly if thinned to far. But even then, "DO NOT THIN" is a little aggressive. Maybe, "Please do not thin beyond x%" or something like that. I have thinned things like shellac, lacquer, and oil-based varnish substantially with no major detrimental effect. Now water-based finishes, those are another story. You really don't want to thin them too far since water isn't actually the thinner. So you can ruin the finish by thinning with too much water.

Sweet, thanks guys.

[PaulMarcel]

I'm with Bøb and routinely thin my solvent-based varnishes 75% solvent (Naptha or Mineral spirits) to varnish. Once I switched to that ratio, I found that the depth of the finish was remarkably better. The dilution ratio carries the finish much further into the pores and deposits a little varnish there. I usually put 3 coats on relatively quickly because I want to stack the coats before the previous coat sets up too much and completely seals the wood. With Seal-A-Cell, that third coat going on is night-n-day difference. After those quick thin coats (first two within an hour of each other, third once the second feels not so tacky), I switch to heavier varnish ratios to get it done faster.

For water-based, like the other said, you'll make a mess of it. I don't have personal experience with it as I never water down water-based finishes, but the chemistry of water-based is that of an emulsion. Anyone who makes their own salad dressing can attest to losing the emulsion if the ratio is wrong.

[Jim Rimmer]

Yeah I believe its a VOC thing as well. Although some finishes will not behave properly if thinned to far. But even then, "DO NOT THIN" is a little aggressive. Maybe, "Please do not thin beyond x%" or something like that. I have thinned things like shellac, lacquer, and oil-based varnish substantially with no major detrimental effect. Now water-based finishes, those are another story. You really don't want to thin them too far since water isn't actually the thinner. So you can ruin the finish by thinning with too much water.

If you thin your water-based finsih you'll have to the the Biblical approach to repair - sand it down, "Repaint and thin no more."