laying up Glass and epoxy before atatching skins

[Iain Hall]

As those who have looked at my build journal will know I have a steel frame and will be cladding it with thin ply (3mm) glued on with Sikaflex then epoxy and fiberglass after much research I have stuck upon the idea of laying up the glass and epoxy onto the ply sheets before attaching them to the frame after they have been cut to shape so they can be done with the sheets flat on a table. I reason that the sheets should not be any more difficult to bend around the curves of my roof(s)* and that the edges and corners where the sides join the roof can and will be joined with tape and more epoxy the "inside" of each sheet will likewise be sealed with epoxy to encapsulate the wood. I do realize that the glass will make the ply a little stiffer but I don't think that the curves on my frame will be tight enough to make bending the sheets too difficult.
So what do the experts here think of this approach?
Can you guys see any major issues with this idea?

This picture should give you some idea of the sort of curves in play with my design as you can see there is nothing with a radius of less than two and a half feet






(*I'm doing a production run of three tears)

[pohukai]

I've built 2 stripper kayaks and have glassed them with epoxy. I glassed my teardrop, but only the roof. Here are my comment:
1) Glassing 1 side stiffens the wood fairly well. Glassing BOTH sides makes it almost impossible to bend. So, what do you mean by encapsulation?

2) Different glass weights will provide different stiffness. I used 4 oz and doubled up in areas needing extra strength.

3) Filling the glass weave with epoxy will also significantly affect stiffness and bending. If you do this, you may want to do this AFTER the roof is attached.

4) If you hear loud POPS when you are bending, that is the glass fiber breaking. That is BAD. Glass is strong in only the direction of the fibers. Pops mean you a weak spot somewhere and that won't be visible.

I glassed my tear drop, but the roof only and edges.

Good luck
Jim

[Iain Hall]

thanks for you prompt reply Jim

1) Glassing 1 side stiffens the wood fairly well. Glassing BOTH sides makes it almost impossible to bend. So, what do you mean by encapsulation?

I am only planning Glass on the outside and just epoxy on the inside

2) Different glass weights will provide different stiffness. I used 4 oz and doubled up in areas needing extra strength.

Fair call I only plan similar glass weight

3) Filling the glass weave with epoxy will also significantly affect stiffness and bending. If you do this, you may want to do this AFTER the roof is attached.

Even with the very modest curves in my design?
I don't think that would be beyond the characteristics of the final composite

4) If you hear loud POPS when you are bending, that is the glass fiber breaking. That is BAD. Glass is strong in only the direction of the fibers. Pops mean you a weak spot somewhere and that won't be visible.

Yeah but the main thing I want from the Glass/epoxy is protection from localized damage from stones/hail ect because the steel space frame has more than enough structural integrity

I glassed my tear drop, but the roof only and edges.

I thought of doing just that too

[tony.latham]

In Steve Fredrick's Teardrop Shop Manual, he glasses the walls before installing them. Makes all the sense in the world.

The top? I'm kind of with Jim on this. I suspect you're concerned with doing it in situ since the front of your drop curves back under.

I've built composite kayaks in moulds and a Freedom 15' stripper canoe. And lots of other projects that involved glassing.

So this is what I think I'd do with the top. (I'm assuming your building a hinged hatch over the galley):
--Attache the plywood to the roof.
--Start glassing from the rear, where the hatch hinge will go.
--Once your to the point of the under-curl, I'd jack up the front as far as it will go and finish the job.

I think that'd work. And of course, attache your plywood to your hatch and glass it on a bench.

I'll be curious to read how you end up doing it.

Tony

[pohukai]

Ok.. glass on one side/epoxy on the other may still allow for flex. The only thing I'd be afraid of is if you're wrong, you have an expensive piece of unusable of glassed ply. Because of the expense, you WILL find another purpose of it somewhere; even if it isn't on your teardrop.

Glassing the sides on a flat surface is a really good idea to avoid drips and sags.

I've glassed enough to know, it is that FIRST layer of glass/epoxy that will determine the overall quality of the effort. Once the glass slides or bunches up, you're in a massive race to correct it before for the epoxy sets up. Filling the weave is relatively super easy at almost any angle and the glass will NEVER bunch up at this point. Mistakes are a lot harder to make at this point.

[Rick Tyler]

Nine plywood/glass/epoxy small boats here. I would never glass then build if you have to curve the panel to make them lay right. Even 4-ounce glass will make it a lot stiffer, and you will get a better bond if you epoxy/glass the roof-to-side joint all at once when you glass the roof. I'm the king of overbuilding, but I would look at adding a layer of 6" wide 9-ounce or even 12-ounce 45x45 biaxial tape over the roof/side joint. It will not leak, and never come apart. If you use heavier tape on your joints, you also will not need any other fasteners along the edges, the glass tape is more than strong and stiff enough for this. A lot of epoxy/glass/ply composite boats have no metal fasteners in the hull (except for things like motor fittings, cleats, etc.)

Full disclosure -- I've never used anything thinner than 4mm, so your 3mm might bend fine. Your mileage may vary, no warranty expressed or implied.

[Iain Hall]

Thanks very much for all of your advice Guys I will take note of your collective experience and do the sides on a table do the roof once it has been installed the hatch likewise on the table after the skin has been attached
thanks again

[Alan_H]

Thanks very much for all of your advice Guys I will take note of your collective experience and do the sides on a table do the roof once it has been installed the hatch likewise on the table after the skin has been attached
thanks again

I'm a little late to this thread, but I think you are now on the right track with this plan. The glass will strengthen your wood and help it keep its shape, but I think you would have a higher probability of failure if you tried to bend it after glassing it.

[Iain Hall]

Alan H

I'm a little late to this thread, but I think you are now on the right track with this plan. The glass will strengthen your wood and help it keep its shape, but I think you would have a higher probability of failure if you tried to bend it after glassing it.

Yeah the more I think about it the more it makes sense

[kerryb]

I'm a little late to this thread, but I'm doing it now at home. You can see pictures in the build journals, "Cargo Woody Tear". I'm not going to glass the curved top before installation because I'm pretty sure that would make bending the ply way too difficult. I will epoxy the inside at installation (today) and glass the outside afterwards. All the flat panels were constructed level on a table to reduce drips and runs.

[Iain Hall]

Looks Good Kerry!

[KCStudly]

3mm is very thin, really, just 1/8 inch (technically less). You could layup a smaller test piece, perhaps 2ft square, let it cure and then try bending it over. I did a test coupon with two plies of 6oz over 3/4 inch foam, about 8 inches wide x 12 inches long, and was amazed that I could still bend it around on itself shortly after the cure. Granted the foam can compress fairly easily compared to wood, but it might work for you.

My bigger concern is how do you plan to create a large enough radius to get the corner tape (cut strips) to lay down well? I have not worked with 4oz cloth, mostly just 6oz. Rutan says you can get 6oz to go over a 3/16 radius (IIRC) or 1/8 inch on a 45deg bias, but I found (at least before I gained more experience) that I couldn't get it to do it. At 1/4 inch I could get pretty good results most of the time without having it pucker back up, at least in some areas (... and then having to sand those area back down and lay another strip on). Yes bias cut helps go over curves a lot, but with just a 3mm joint you won't have much material to round over. Should be easier with 4oz, but you should still test.

Maybe a flox corner? But then again I'm guessing the radius on your steel tube is probably only about 1/8 inch to begin with, so not much to work with.

Glass fibers don't like to bend over sharp corners. You'll think you have it tucked down tight, only to come back and find that the fibers have tried to straighten out again half way thru the cure. This results in a blister, or bubble of air under the cloth on one side or the other next to the corner where the cloth springs up away from the flat to make its own radius.

Have you done any test pieces to prove that you will be happy with just one 4oz ply over the 3mm? When I tested the 6oz directly over foam I decided I wanted a little more strength and moved up to two plies. Just saying, test.

And I will add, when testing make sure that you do a big enough test to be sure that you don't have scale up issues. For example, I used a narrow strip of scrape 5mm ply as a story stick to layout my front roof panel and test bend to my front radius. The narrow strip formed right around the tightest radius with very little effort conforming to the curve well; but when scaled up to the full width panel it took a lot more effort with strong backs, straps and clamps to get the ply to conform. I know this is kind of an obvious example, but it is something to keep in mind... the scale up.