I built some benches, they're pressure treated lumber currently finished with oil based stain and oil based satin finish spar urethane. Some of them have shellac between the stain and urethane because I didn't have time to let the stain dry.
They're public benches and one of the problems I'm seeing is when people set a cigarette on the bench it burns the spar urethane, which bubbles then flakes right off. The stain underneath seems to hold up fine though.
My question is: Is there a clear finish that can go on top of spar urethane, or instead of it, or something I can mix with it, that will prevent or reduce the damage from cigarette burns?
I'm not talking full on flame proofing these, just something to protect the spar urethane from high localized heat for short periods of time.
Ideally it would also hold up as long or longer than the urethane, it's outside with heavy exposure to the elements.