Heat Damage to a Shellac and Polyurethane Finish

Shellac has limited tolerance for heat. In this case, a laptop computer apparently damaged the shellac undercoat. April 17, 2010

Service call: Customer reports finish crazing on a veneered tabletop.

The table is finished with polyurethane over shellac sanding sealer. Veneer is pomelle sapele vacuum pressed with urea formaldehyde (Unibond 800) glue. Substrate is an MDF and honeycomb torsion box. The bottom is veneered and finished.

In two spots there are 4" diameter areas of crazed finish. I also noticed L shaped impressions in the finish very near the crazed areas. I asked the customer if the areas had been exposed to heat. "No, of course not."

My hypotheses: The heat from the laptop fan after hours of use (which I can feel on my leg as I type this) has caused the finish failure, the L shaped marks being the feet on the laptop. The customer does admit he uses his laptop on the table.

Have y'all ever heard of this happening? Is there a furniture finish that would stand up to the temperature changes caused by this use?

Forum Responses
(Finishing Forum)
From contributor B:
I don't think oil finishes will resist this level of heat. You need a 2 pack finish. We call it acid-catalyzed; maybe you call it conversion varnish. I have done restaurant tables which take hot plates and alcohol without marking. You must strip off the polyurethane and start again. Don't do it for nothing - it's the customer's fault.

From contributor C:
The weak link in your finish is the shellac. Sanding sealer - do you mean a shellac that has a sanding aid added like a nitro lacquer, or just wax-free shellac that you're sanding after drying?

Shellac and heat don't mix well, especially in long term contact in the same spot. You need do no more than remove and give the surface two coats of the polyurethane instead of 1 shel/1 poly.

Never use shellac on a surface that is going to possibly receive heat to an extent where it will affect the shellac.

From contributor J:
I agree the shellac is the weak link. Assuming that this was a new work situation, the shellac is uncalled for, given its shortcomings in the resistance department. I think shellac as a seal/barrier coat should only exist in the re-finisher's finish schedules.