I would normally use shellac but have found it takes too many coats to get a thick covering on the surface. The waterborne urethane varnish I have started using goes on thicker so only one or two coats are needed to stop any excess paint penetrating the top surface. And being water based, the dry time is only an hour or two.

Also I was told that if I use shellac it has to be fully cured otherwise the metho will react with the water based paints and cause cracking issues. So even thought the shellac dries quickly, I would still be forced to wait for a day or at least several hours. So that defeats the purpose of using Shellac.

I will try another sign in the next few days but this will be MDF and have a painted surface and contrasting painted engraved section. I only sand back the vanish enough to get rid of any unwanted paint that was used in the engraved areas. Once that paint is removed, i will spray the whole sign with the varnish to finish it off.