Do I need to use Fiberglass mat, or just Resin?

[smahlberg]

I have a 15 ft. 1980 Tuffy fishing boat. I am building new casting decks out of 3/4" treated plywood.

Do I need to use Fiberglass matting or can I just roll on a couple layers of epoxy resin to seal it. I was only going to use Fiberglass matting to join old wood to new wood. Otherwise I was just going to roll on the resin.

When is Fiberglass matting used and what are the benefits?

I don't see the reason to use fiberglass matting unless you are reinforcing a joint, crack, imperfection, etc.

[TripleCrownNC]

You will never be able to seal the edges with just resin. Mat really has no structural strength, but it does absorb a lot of resin, so its used as a first layer in a layup of several layers. Mat then structural glass like 1708.

Guessing at what you are doing, I would miter the edges of the ply 45 deg, round off the corners some, apply resin, then a lyer of mat, wet out, then a layer of 6 oz cloth to give it a clean look. The miter and rounding are so the mat and cloth will lay smooth.

[apex1]

That was wrong Triple, sorry!

If we are talking about Epoxy (and we should, `cos polyesther does´nt stick to wood), he does not need anything else than the resin.
smahlberg.......Endgrain has to be covered several times to encapsulate the wooden fibres. If the surfaces is painted (not varnished), you may apply one layer of plain resin and another one of EP putty to close the endgrain, then another layer resin to close the (sanded) putty surface. On the ply surface you need two to three layers of epoxy resin to make it waterprotected. Then paint it to protect the resin from UV rays! Mat adds weight and cost, nothing else.
Regards
Richard

[smahlberg]

Thanks for the clarification. So two or three layers of epoxy resin will be enough to seal the plywood? I will be carpeting over the decks, so no need for paint.

Can you think of any circumstances where I would need to use mat?

[alan white]

All to seal plywood, but sometimes it can make sense to use, for example, cleated cedar boards, which need nothing more than a coating of oil for protection, and which cost far less than good plywood and epoxy and glass.
This applies if the area beneath the deck needn't be sealed or covered from spray or rain.
Fasteners would be stainless screws. I am adding this comment because it seems that way too much epoxy/glass is being promoted as the only way to feel good about the job, and sometimes that is true. But it is also true that sometimes epoxy/glass is a big waste of time and effort. With a gallon at over a hundred dollars and a quart often not enough to do the job, buying another quart because you ran short (plus hardener) becomes even more costly.
A cedar deck would need no non-skid either, nor primer and paint. 1 x 6 western red cedar costs about $4.50 per sq ft. A quart of Deks olje or similar oil or protectant maybe $20.00.
There is also the question of the use of pressure treated plywood as a substrate suitable for applying epoxy. Supposedly, pressure treated wood is already ready for the marine environment and a lot of boat builders use it for applying carpeting, without any treatment whatsoever.
I sure wouldn't use PT plywood if I was planning on encapsulating it in resin.
I'd use it because it was going to be left unprotected!
Also, it is never wise to coat only a portion of a piece with epoxy. you aren't keeping the piece from rotting if moisture can sneak in from underneath and get trapped under the glassed and resined top covering. so there's more expense.
My recommendation would be to assess whether you need the deck to become a permanent part of the boat to begin with. A removable deck allows repairs and inspection later. That's a good reaqson to use either solid wood slats with spaces between with cleats or perhaps pressure-treated plywood with carpet stapled onto it. Or, for real simplicity, just pressure-treated plywood with porch and deck enamel/non-skid over it (the paint sticks fine).
I would not use glass/epoxy unless the deck were permanent, as a part of the boat, and since the boat needs no more structural members and parts, it's far better to tab in some cleats to make a basis for screwing the ply down. Or, solid wood if there's no need to keep water out (Always go for extra ventilation if you can aboard a boat to prevent mold and the rot that inevitably follows.

[TripleCrownNC]

Apex, there was nothing wrong with what I said. Seems to me that glassing over wood has been going on a year or two.

If you like, go ahead and not glass it, cover it with carpet. You learn by doing.

[alan white]

Much depends on the desired end result. So many factors are involved that no one way is best in all situations. I go with Apex if the boat is going to be sealed up below the deck. Simply sealing with epoxy (all sides) will ensure no rot is going to take hold for a long time. However, Triple may be anticipating voids in the plywood which would, if not rot easily, degrade the surface down the line by collapsing in the surface veneer, creating, in essence, a hole.
All things considered, better plywood would do well just epoxy-sealed and painted. Cloth would be redundant unless very heavy use (traffic) was anticipated. One has to weigh these factors.
Marine plywood is not cheap, but 3/4" ply isn't needed either. 3/8" would suffice in a light boat if framed under every 16" or so. Weight matters and 3/8" weighs half of the weight of the PT ply. Or again, see if cedar boards (or fir or cypress, etc.) will do. They need neither epoxy or cloth to last for 20 years. Just oiling and ventilation.

[smahlberg]

Thank you to all for the advice.

[apex1]

Originally Posted by TripleCrownNC

Apex, there was nothing wrong with what I said. Seems to me that glassing over wood has been going on a year or two.

If you like, go ahead and not glass it, cover it with carpet. You learn by doing.

Would´nt offend you, and carpet was not mentioned in the first post. But Epoxy covered wood does not need any mat for any reason to make it waterproof.
So, this statement was definetively wrong
"You will never be able to seal the edges with just resin"
Regards
Richard