What is a proper grit to prepare "A" faced ply (birch if it makes a
difference) for epoxy coating prior to painting.

Ron

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?

"JS" writes:

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

I don't have a clue. I described what works for me.

YMMV



--
Lew

S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: http://home.earthlink.net/~lewhodgett for Pictures

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.

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!!

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

(Ron Thornton) wrote in message ...
What is a proper grit to prepare "A" faced ply (birch if it makes a
difference) for epoxy coating prior to painting.

Ron

Sorry I did not read this post and just responded to the end of the
post. I addressed glueing stuff together in another post but I did not
answer your question.
I would probably use something like 150 to prepare for the epoxy. Put
your goo on real thin the first coat, and sand and apply another...
very thin, no fillers, this should give you a nice surface to paint
on. Then wash off the blush (wax), sand with 250 and have at it. Most
paints will go right over most epoxy without primer in my experience
but the blush must be all gone, check with the manufacturers too of
course.
Scotty
I should note that I am not a fan at all of so called "encapsulation"
and have no time to address that here, but I do often use epoxy as a
"sanding sealer" and then primer as described above. This is for me
an economics thing as I get my epoxy pretty cheap, cheaper than paint,
sealer, or varinish. I can also count on a solid cure in a determined
period, something the others can't always offer me.
"Effin'A"