Repair vs. durability

One of the most often heard complaints about most modern finishes is the difficulty of repairing them.

Modern finishes like catalyzed lacquers and conversion varnish do not dry in the traditional sense. They set or cure. I realize that these terms are subject to interpretation. But traditional finishes dry through the evaporation of the solvent components, while the newer finishes dry through a chemical reaction.

There is little doubt that these newer finishes are much tougher than traditional finishes, many of which have been notoriously easy to damage. Anyone who has spilled a drink containing alcohol on a shellacked surface will tell you how easy that is to destroy. Nitrocellulose lacquer can get pretty hard but can be easily scratched and is susceptible to water damage.

On the other hand, the characteristics that make traditional finishes easy to damage also make them very easy to repair or restore. Shellacked finishes can be brought back to life by applying a fresh coat. It might take some work to get the sheen to blend with the old but it’s completely doable. And lacquer finishes can sometimes be restored simply by applying a coat of lacquer thinner.

Anyone who is given the job of repairing an old or damaged finish will tell you that they are very happy when they find out they will be dealing with lacquer or shellac. “Poly” and catalyzed finishes are much more difficult and often impossible to repair. They usually end up being completely stripped and refinished.

D.D.

COMMENTS

  1. Tim Neun wrote:

    Hi David,
    I need to clean up a dining room table top that was finished with precat lacquer. My plan was to sand it throughly but not completely remove the old finish then spray additional coats of precat lacquer. Am I rapidly heading for disaster? Wiil the new finish bond with the old or shall I just sand all the old finish off & start fresh?
    Thanks for your help,
    Tim

  2. Keith Allen wrote:

    You need to go back and read some of the articles in Woodshop News that accurately describe classification of finishes. Shellac and “ordinary” lacquer ARE “solvent-based” finishes, which don’t change chemically when they dry (i.e. when the solvent evaporates.) But “ordinary” varnish DOES change chemically when it dries, namely such finishes OXIDIZE, which IS a chemical reaction. Many of the latter CAN be repaired fairly easily, by removing any wax present, lightly sanding damaged areas, and re-applying the original varnish, or something similar.

  3. Mark Slafkes wrote:

    I believe David was trying to describe finishes where the molecules simply stay unlinked, like shellac, vs finishes where the molecules link together or undergo substantial changes. By the way, ALL finishes that I know of utilize solvents. Water is a solvent as are the more volital chemicals such a mineral spirits, lacquer thinner, toluene, etc.
    Mark

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