my copper is 22ga thick when I start etching. I  use the unleaded Thompson enamels, because I like the transparent colors that can be used directly on my etched design. I prefer to wetpack an even first coating. It is importend to allow enough time, say 2 1/2 min. at 1450 degrees to absorb the cuperous oxide at this first firing. the piece should be clear and bright and the etching visible.   Following coats can be fired at the normal 2min. until finished.( counter enamel is applied first .                                                                                                                              )To see a step by step direction of this method check out the "Art of fine enameling"  by Karen Cohen where my chapter is on Basse Taille.                                                   The resist for ferric chloride that i use now is PnP transfer paper versus asphaltum  direct, which is very messy!