Q: I’ve stained and sealed with Zinsser SealCoat and plan to glaze with a Van Dyke oil-based glaze. I wanted to top with Zar Interior Polyurethane antique flat, but Zar says polyurethane will delaminate off either the glaze or SealCoat, which has no wax, but may contain stearates. Now what?
A: First, SealCoat contains no stearates and is perfectly compatible under both oil based and waterbased polyurethane. Second, oil based polyurethane will bond just fine to oil based glaze. The problem, and it has nothing to do with Zar or any other polyurethane for that matter, comes if you leave too much glaze on the surface, or if you mix the glaze with insufficient binder.
Whether a coat adheres to the previous coat is only half the problem. The other half is the integrity of the middle coat itself. If you create a glaze made of pigment with so little binder that when it dries, it acts like a barely held together layer of pigment powder, it will be the weak layer in your finish. Even if finish sticks to the top of this glaze layer, and it sticks to what is beneath it, if the layer itself breaks apart, you have delamination.
The key, then, is either to make sure you add very little glaze before sealing, and you can do that by glazing several times if need be with sealer between each application, or use a glaze that has enough binder to maintain its own integrity as a coat of finish.
Michael Dresdner
straight talk about wood finishing
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