I don't know about the dewaxed or not, but I use the premixed Bull's
Eye shellac available from home depot. It is already mixed in a
factory, so they can do it much more accurately than I could ever. It
is dewaxed, and can be thinned with alcohol if necessary. It may be
hard to find in Mexico, but I bet that you can get it by mail. My
guess that the hardware store shellac might have used to be passable,
but it has probably sat there much beyond its shelf life, which is
usually only about 6 months.