strotolite (Probably wrong spelling) it is commonly used for the scratch coat
over lath for true plaster walls. Put a peice of plastic drop cloth 2x the size
of the tub base under the tub add the mud then fold the plastic over the top of
the mud. keeps it from sticking to the bottom of the tub . dont stand in the
tub till it has set or you may get a gap and not get full support.