I was using clear shellac (Zinsser) on some home center-bought red oak, intending it only as a “sanding sealer” coat. It went on as expected, with no apparent color to the shellac and no impact to the wood color. But then I noticed as the pieces were drying that the oak was taking on a dark tone, brownish on some pieces and more gray on others. I was expecting the oak to keep its natural color. Any idea what caused this? I’ve never seen clear shellac change wood color like that. The shellac was a partially used can I had on hand and I don’t know how old it was. I looked for an expiration date but could only find a “lot number” with no obvious way to deduce a manufacture or expiration date. – Henry Burks
Tim Inman: Oak contains lots of tannic acid. The solvents carrying the shellac are usually pretty basic. My thinking, without seeing your problem, is that the acids are reacting with the bases in the shellac and graying out your wood. “Homemade” shellac using denatured alcohol would not give this reaction so quickly. Also, avoid drowning the wood with wet solutions of your shellac. Use thin coats and let the solvent flash off quickly. That way there is less time for the acids and bases to react and spoil your project.