Glassing Plywood

Hi, I'm working on a hollow wooden paddle board (a really long surfboard designed just for paddling) and I'm having some issues. I've been reading posts about marine plywood vs exterior plywood and I understand marine plywood is stronger and more rot resistant. What I don't understand is if you glass the plywood why would it matter if its rot resistant or stronger, isn't the glass providing a huge strength component and making the entire thing completely water proof?
I've attached a picture of what I've done so far, I intend to put plywood on the deck and the bottom and leave the rails exposed (the frame and rails are cedar) then stain it and glass it.
I'd really appreciate any help. Thanks

 

The biggest problems I have with regular ext plywood involves lack of quality control, i.e. unseen voids in the middle layer, and very thin outer layers.
For something that small, seems to me it wouldn't be worth the trouble of using ext plwood. How much are you going to need 2-3 sheets? That's just not that much money for something that you obviously have done a good job on so far.

 

Fiberglassing the plywood (with epoxy) will "encapsulate" the wood and give it some additional stiffness and abrasion resistance

If there are voids in your wood - then putting some glass on the out side is not going to help much.

 

Go for a WBP exterior ply (Water & Boil Proof). The stuff I've seen here in England anyway, seems decent, knot free and is seriously cheaper than marine ply.

 

In a perfect world, you can epoxy and 'glass toilet paper and everything would be fine, the 'glass provides the strength and epoxy, the water tightness. In the real world, the 'glass skin gets nicks, scratches, has minor imperfections, unseen pin holes and other, possibly damage related ingress points that would turn your toilet paper core, into goo pretty quickly.

This is the problem with 'glassing anything, the coating WILL get breached at some point. This is particularly true of a beached craft, or one that will get dragged over sand bars or other, sheathing unfriendly places.

Marine grade ply will provide you extra protection and have a higher quality of construction, within the panel. Marine panels will bend uniformly and have rot resistant species used throughout the panel.

This said, I know several people that are building small boats (kayaks and canoes) using Lowes/Depot lauan. These boats get treated well, are covered and have lasted for several years.

If you take care of the boat, keep it clean and covered, in a well ventilated area and keep up with any maintenance issues, like quickly repairing dings, scratches and the like, you'll be fine with a cheaper grade of plywood. The cheaper grade of plywood will be weaker, more prone to rot, will not bend as nicely and generally prove you get what you pay for, but if it works for you, then go for it and build your boat.

Given a choice, between no boat and one of construction grade plywood, then build the boat with what you can get and deal with the related issues as they come up.

 

thanks for the advice guys. i think i'll probably use exterior grade and a nice glass job, marine plywood is unavailable here and I'd have make a 6 hour trip to pick some up. I'll be sure to be vigilent about repairing dings to keep the stuff from getting wet.

 

My experience with small, sheathed-in-fabric-and-epoxy plywood craft is that interior grade cheap ply gives you a useable prototype that will last a season or two, good exterior grade ply will make a boat that will last ten or fifteen years with good care and maintenance or five to seven years of abuse, and high-quality marine grade ply will make a boat that, with good care and maintenance, can be willed to your grandchildren.

 

You might find this site interesting:
http://www.laughingloon.com/epoxy.html

 

Marine grade plywood

Hey Tie phoon:
You don't have to travel 6 hours to get good wood... You can have it delivered to your doorstep. Like par said, you get what you pay for. If your not intending your craft to last more than a few years, then go for it (the cheaper wood that is). If your interested in something that you might want to will to your kid or grandchildren (as mentioned by MMD) by all means go for the good stuff. Boulter can give you what you want and deliver it to your doorstep via UPS or whoever. I live in central Illinois and I guarentee you that NO ONE WILL SELL MARINE GRADE PLYWOOD around here. I was very satisfied with my wood and their service. If you wish, I can get you their # and or web address.

Best of luck.............

 

Try www.woodfinder.com . . .

 

A suggestion.

The glass will add a lot of weight that won't really be structurally necessary.

Racing dinghies from the plywood era were never glassed all over and rarely epoxied (though it does make them better in a maintenance sense - so I would support that suggesion).

I would just epoxy coat it and consider glass taping the bottom chines if you are worried about abrasion - but that is even not necessary as I have found with a whole bunch of wooden windsurfers, surfskis and racing dinghies.

Best wishes
Michael Storer