It is best to use a hard wood species with very little open graining rather
than soft. I agree with Iris, acrylics are the best for wood. There is a
possibility that the wood resins will also create blank "bubble like" areas
in the pattern. Try wetting the wood first (do not soak), let it dry, sand
well to remove the raised fibers, then apply the alum solution and let it
dry again before marbling. Also take into consideration the amount of
"pattern" you will need to completely cover the wood surface. If you use
the same technique as when lowering paper to the surface...you are bound to
get "air pockets" as a result. Try lowering the wood at an angle. This is
tricky...practice on small pieces first until you get a pleasing "all over"
pattern without flaws.
Good luck! -Milena