I've cut the deck open, cleaned out the mush, sanded,, bought new balsa..

Ok,, now I must put glass on top of the balsa.. build up to level with
existing deck..

What cloth to use? What is biaxial cloth?

I figure two to three layers should do it ..

do I start with one type, lay another type on top,, and then another to get
a
kinda smooth surface?

See:

http://www.westsystem.com/ewmag/19/Replacing_Core.html

MW
Los Angeles