I don't like using shellac on the back with dry heat. I've found that it traps the heat in too well and you can easily get little cracks, from pressure build up of hot air trying to escape from the cells.
I prefer to use wood glue on wet wood, with steam as the glue plasticises.
On dry wood with a hot gun, I use cooking oil. No dry coating of shellac or glue.
Hamish.
