I find that I have more success in shaping the mokume if I start with
a thicker piece - 18 to 20 gague has worked well for me. I shape my
pieces using dapping blocks and punches, or rawhide mallets and wooden
forms. Beginning with a thicker piece has allowed me to do more
finishing of the shape and remove any imperfections in the surface.
When I began, I tended to roll the metal out too thinly and so I
didn’t have enough thickness to sand and finish properly. I am not
working with gold or shakudo (I work mainly with billets of
copper/sterling or copper/brass and nickel) but I have been able to
make all sorts of shapes out of my mokume.
If the billet is sound, using thicker materials should solve your
problem, I think. If this seems like a waste of material, you can
also solder another piece of metal to the mokume and use the mokume
like a veneer. This increases the problem of marring the surface but
does spread the mokume out a bit.
Finally, one of my first problems was falling in love with a
particular pattern on the sheet, incorporating it into a piece and
then being unwilling to sand or polish out any imperfections because I
would lose/alter the pattern I loved. I just finally got over it and
my work is better now.
A good reference for mokume is Steve Midgett’s 2000 book Mokume Gane.
A Comprehensive Study. Earthshine Press. ISBN 0-9651650-7-8. I
ordered my copy from Reactive metals.