JS wrote:
Now it is time to lay glass.

Haven't had a wood/glass bond let go yet.


I take it that glass directly on wood doesn't hold well? Why doesn't
it adhere as well without glass as with?

It is better to put down a coat of epoxy first to fill pores so they
don't drag resin out of the glass. Epoxy adheres to most plywoods
extremely well but a light 2oz scrim of glass keeps the face veneers of
the plywood from checking and makes a more stable surface for paint.

AS to the original question, it depends on what you plan to do but the
grit for birch is not all that critical. For a clear varnish top coat I
would sand to the birch to #120 grit, apply the epoxy and scrim, fill
the scrim and sand to #180 grit. If using a marine paint I would go to
#80 and #120 respectively then apply the recommended high build primer
and sand to what ever the paint system recommends.

You would need to stay with a coarser grit for tougher to bond woods
like white oak and teak because you have to rely more on the mechanical
surface bond.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com