Michael Dresdner

straight talk about wood finishing

Apples to apples?

Q: We want to do a comparison of the performance of a variety of solvents for shellac:Lee Valley’s Shellac/Lacquer Thinner; a lab grade of denatured ethanol and common hardware store methyl hydrate. We plan to prepare samples of blond shellac, mixed in each of the three solvents, apply them to wood samples and test the resulting finishes. I’m looking for information on what kind of tests are used in the industry to evaluate the performance of shellac finishes. I’m interested in time required to recoat, sanding ability, and final resistance to abrasion and liquids.
A: Pure methanol (what you call methyl hydrate) is considered the traditionally ideal solvent for shellac. However, because it is so dangerous, vastly safer ethanol has long been used, and is used almost exclusively here in the US. Fortunately, the difference between the two as a shellac solvent is almost indistinguishable.
Which alcohol you use, and whether you use pure alcohol or a mixture of alcohol and other solvents, will determine how fast the flakes dissolve, with pure ethanol and methanol being the fastest. In addition, higher molecular weight alcohols, such as propanol and butanol, will require more solvent per resin weight. As a result, adding a given amount of shellac resin to the same volume of, say, ethanol and butanol, with result in the butanol dissolving slower, and will also result in the butanol yielding a slightly thicker, more viscous mixture.
Drying time will be determined by the evaporation rate of the solvent. Again, ethanol evaporates substantially faster than butanol.
As for the final finish; if the shellac resin is given enough alcohol, whether in mixture or alone, so that it can be fully dissolved, and if the mixture is given sufficient time to dry, which of course will vary with the solvent evaporation rate, the eventual film formed will be the same. That’s because once the solvent evaporates, you are left with pure shellac resin in a thin film on the wood. In terms of ultimate durability, how that film of resin got there or how long it took to dry are of no consequence. Thus, a test of the final finish will give the same durability and sandability, though recoat time will depend on the solvent evaporation rate.
By the way, if you add oil to your French polish pad, that, unlike the solvent choice, does indeed alter the final film. Most oils make the shellac film less brittle, weaker and more prone to water spots.