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

Glenn Ashmore ) writes:

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.

I've read that the reason epoxy bonds well to wood is that it bonds at the
molecular level, the mollecules of curing epoxy being the same size or
something as the molecules of wood. So long as the surface of the wood is
clean and dry it doesn't need to be sanded for adhesion, except as noted
above in the case of woods with incompatible resins. Otherwise you only
sand if you want a smoother surface for the finish.

--
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(William R. Watt) wrote in message ...
Glenn Ashmore ) writes:

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.

I've read that the reason epoxy bonds well to wood is that it bonds at the
molecular level, the mollecules of curing epoxy being the same size or
something as the molecules of wood. So long as the surface of the wood is
clean and dry it doesn't need to be sanded for adhesion, except as noted
above in the case of woods with incompatible resins. Otherwise you only
sand if you want a smoother surface for the finish.

While this might be true at some level, I always rough up the surface
too. Can't hurt to add a little mechanical bond to that molecular
bond..
Scotty from SmallBoats.com

On 8 Jul 2003 14:37:46 GMT, (William R.
Watt) wrote:

Glenn Ashmore ) writes:

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.

I've read that the reason epoxy bonds well to wood is that it bonds at the
molecular level, the mollecules of curing epoxy being the same size or
something as the molecules of wood. So long as the surface of the wood is
clean and dry it doesn't need to be sanded for adhesion, except as noted
above in the case of woods with incompatible resins. Otherwise you only
sand if you want a smoother surface for the finish.


I don't think that is quite right, as in molecules bonding?
The word attraction, or resistance to surface tension, may be the
better term. - So you looking at wetting agents, and surface tension.

As in bonding, - would mean the epoxy is disintegrating, or migrating,
as in solution - which is want you wouldn't want.
What you would want is an ability to penetrating through the cell
walls. - As in its saturation ability.
In other words the chains can slip through the holes, and fill up the
cavities, so you wouldn't want, oils and waxs in the timber, but it
can take in a bit of water, so a small amount of water in the timber
as a wetting agent can help its movement.

If you try to separate two layers, you will see as it tears out the
fibers, as to its penetration ability - and you could test on various
formula -( lack of fillers ) As the timber is seen as just another
fiber type filler.
As for sanding, its not that big a deal, - but that's relative to the
saturation point of your mix - and any fillers then added.- the more
filler the more sanding, the less saturation.

wrote in message . ..
On 8 Jul 2003 14:37:46 GMT, (William R.
Watt) wrote:

Glenn Ashmore ) writes:

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.

I've read that the reason epoxy bonds well to wood is that it bonds at the
molecular level, the mollecules of curing epoxy being the same size or
something as the molecules of wood. So long as the surface of the wood is
clean and dry it doesn't need to be sanded for adhesion, except as noted
above in the case of woods with incompatible resins. Otherwise you only
sand if you want a smoother surface for the finish.


I don't think that is quite right, as in molecules bonding?
The word attraction, or resistance to surface tension, may be the
better term. - So you looking at wetting agents, and surface tension.

As in bonding, - would mean the epoxy is disintegrating, or migrating,
as in solution - which is want you wouldn't want.
What you would want is an ability to penetrating through the cell
walls. - As in its saturation ability.
In other words the chains can slip through the holes, and fill up the
cavities, so you wouldn't want, oils and waxs in the timber, but it
can take in a bit of water, so a small amount of water in the timber
as a wetting agent can help its movement.

If you try to separate two layers, you will see as it tears out the
fibers, as to its penetration ability - and you could test on various
formula -( lack of fillers ) As the timber is seen as just another
fiber type filler.
As for sanding, its not that big a deal, - but that's relative to the
saturation point of your mix - and any fillers then added.- the more
filler the more sanding, the less saturation.

I guess I was more trying to make a point with my simple terms. That
point being, I always take an extra minute to rough the surfaces up
too, especially in structural areas. For areas that will be hidden in
a joint or filled later I use two tools for roughing. One is a heavy
grit offset grinder and when I can't get that in there or for more
precice work I take an old jigsaw blade, add a little bend and draw
the blade over the surface in a cross hatch pattern. And just to take
it one step further, I always mix up a small batch and throughly wet
each surface then add filler to the rest of the batch and paint or
spread it on the wetted surfaces then fasten or clamp.

I have always reccomended using as little filler as necessary to hold
the goo in place. A lot of my construction calls for epoxy to act as
as adhesive (glue?, simple terms again) as opposed to just filler for
glass... I have always explained it as "let the epoxy do what epoxy
does"... anyway simple terms again, but now I have a better
understanding as to how it works.
Thanks, Scotty

(Backyard Renegade) wrote:

I guess I was more trying to make a point with my simple terms. That
point being, I always take an extra minute to rough the surfaces up
too, especially in structural areas. For areas that will be hidden in
a joint or filled later I use two tools for roughing. One is a heavy
grit offset grinder and when I can't get that in there or for more
precice work I take an old jigsaw blade, add a little bend and draw
the blade over the surface in a cross hatch pattern. And just to take
it one step further, I always mix up a small batch and throughly wet
each surface then add filler to the rest of the batch and paint or
spread it on the wetted surfaces then fasten or clamp.

I have always reccomended using as little filler as necessary to hold
the goo in place. A lot of my construction calls for epoxy to act as
as adhesive (glue?, simple terms again) as opposed to just filler for
glass... I have always explained it as "let the epoxy do what epoxy
does"... anyway simple terms again, but now I have a better
understanding as to how it works.
Thanks, Scotty

I read all these sort of things years ago when I started using epoxy
and so I did some tests. I made a series of lap joints where the lap
was four times the thickness of the wood. I used mahogany, red oak,
and fir. I made pieces planed smooth, fine sanded, rough sanded, and
rough sawn. Finally, I tried tight clamping, and loose clamping. I
did coat the pieces with unthickened epoxy and allowed them to soak up
for 10 minutes before adding thickened epoxy and clamping. I was
surprised and pleased to find that in all cases the joints were
stronger than the wood.

After having accidently left a block of wood next to a puddle of
penetrating epoxy (Syatem Three Clear Coat) one day, I found the next
day that it was bonded to my work bench. So I experimented and found
that I could even make a good joint by clamping pieces of smooth and
flat wood together dry and dribbling Clear Coat on the edge of the
joint, letting it suck in by surface tension.

So, using clean wood and good epoxy, well mixed, I don't worry about
how smooth the surface is as long as I have full contact in the joint.
With glass over pre cured epoxy, it generally has to be washed to
insure you don't have an amine blush which will interfere with
bonding. Sanding won't remove the blush, only spread it around unless
you remove the whole surface. The same care should be taken with any
epoxy joint over pre cured epoxy.

Al

How's the boat coming, Al?
Darn, I like that design.

Jim

Al Gunther wrote:
snip

I have found that if you REALLY want something to stick and it's
important, it will not stick. However, all those little drips and gobs
that seem to end up where you don't want them tend to be there
forever...go figure!!