Polyurethane varnish is normally quite expensive, has poor filling properties, long curing time (12 hours between layers I think it was) and is potentially more hazardous to health than regular dope.

On the other hand, it is very resistant to scratches and most chemicals.

I have used it for fuel-proofing with great success. If thinned, its easily sprayed. You can also brush it on, however, the result will be heavier and not as good looking.

Polyurethane paint is excellent on top of fiber-glass mouldings.

/Bo