layup technique and resin question

[Olas]

Hey All,

I am looking at my first board build. I've got a few ideas for shape and I think I will be focusing on a higher wind/smaller skimboard as the production skimmer I own works well for light wind.

I want to make a thin board while maintaining stiffness and have a decent single concave. I'm looking at taking 3 sheets of 3/16th plywood and laying the first on a basic rocker table. I will coat/impregnate it with a generous amount of epoxy resin then place a sheet of precut glass cloth and then the next piece over top. Repeat this process with the 3rd piece and then weight it down to put rocker/concave into it.

I am planning on using the same cloth and epoxy to do one layer on the top and bottom of the board to seal it afterwards. I am hoping that if I precut the board shape and cut the cloth about 1/8” larger than the shape I will have enough resin “squish” out and harden that I will be able to use this to shape the rail out of. Hopefully I will be able to get a very hard rail and less chance of a waterlogged board.

Anyone have any comments/criticism on my proposed method? I am a total beginner to both wood working and shaping so any advice would be welcome. I am hoping the extra epoxy held by the glass cloth will give the board extra stiffness while keeping it thin, however I am worried how well the resin will bond the wood to the cloth.

I was at the hardware store today and was looking at resin and could only find the BONDO brand stuff in reasonable amounts and I believe it is a poly resin not epoxy, which is bad...maybe? Can you guys point me in the right direction for epoxy resin in terms of what to look for a a rough estimate of the amount I will need? I'll overshoot a bit to avoid running out at a critical time!

Thanks,

Olas

[tungsten]

Hi Olas,

good thing to make your own rides, congrats! Just a quick couple of comments on your plans before you start, because it won't work as you thought it out. Lots of the stuff you mention were exactly the first mistakes I made, so you can spare that step and start by doing it properly

I will get back to you later with a list of tutorials how to do it properly. Worth the while.

Comments:

Resin:
-I would go for epoxy, not poly. Mechanical properties are lots better. Which translate in stiffer lighter board.
-You need roughly 1 to 1.2kg per board. Buy 5kg for a couple of boards. Never mix more than 100g (3OZ) at a time. ALWAYS use protection, touching uncured epoxy can cause allergies which you'll have for the rest of your life
-Either buy the expensive epoxy from nautic shops (West System et al.), or buy the budget epoxy which is used for floors and swimming pools. Hardware stores usually don't have resins, search for specialized stores online, or call the swimming pool / floor company in your vicinity. The cheap stuff (5 to 10 times cheaper than the nautic stuff) works the same, usually it has no UV protection and yellows out in the sunlight. I always use the budget stuff.

Construction:
-Layers: if you want to bend ply over a rocker table, it's not supposed to have concave. You cannot bend ply in both axes at a time, not even thinner 3mm ply, not to speak of the 5mm (3/16") you want to use. Unless you have a 100 ton press and steam, that is You'll have to shape your concave afterwards.

-Weight: with 3 sheets of 3/16 ply, your board will be VERY heavy, like 5kg. I would go for 3mm top sheet, 5mm bottom sheet (which you shape to concave), and 2x 5mm sheets in between, which you reduce in weight by cutting away 70% of the material in slits parallel to the stringer.

-Glue: Using epoxy as a glue is possible only if you are vacuum bagging. Weighing it down, only a very small part of the surface will have bonding contact. Epoxy is also very heavy with 1.1kg / liter, and wood soaks it up. I therefore recommend PU wood glue (like gorilla glue); PU forms a foam and fills gaps up to 1mm; slightly dampen the surfaces before you apply the glue, PU needs a bit of moist to bond.

-Weighing it down: You need lots of weight, like 80kg, or lots of clamps holding down the board with a bar every 4 inches, if you want a good result. Get prep and plan your rocker table strong enough.

-Glass: glass in between the sheets adds only weight, not stability. Pointless.

-Glassing top and bottom: 1x 4OZ top with a second patch under the feet, and 2x 4OZ bottom will do. Sand your top / bottom sheet with 180 grit before glassing, it will suck a lot less resin that way.

-shape and glass the rails as I pointed out here: viewtopic.php?f=1&t=911&start=30

I'll put up a list of tuturials later where you can watch people working those things, or you start searching youtube. There are plenty useful tutorials on there, search "board building".

later, tungsten

[Olas]

Tungsten,

really appreciate your advice, you saved me a lot of time and frustration! I've read a lot of the different builds on this site and thought my method would be a shortcut to a reasonable, albiet not ideal final product. Did not realize plywood could not be flexed on two axis! I was already planning on basing my rocker table on your specs, however I had no idea the forces involved. What do you use to remove wood material from the bottom of the board?

I will use the method you linked to for rails as well, I was trying to put a TT type rail on the skimmer however if I use glue to hold the board together vice epoxy i don't think it will work.

Thanks for your advice on the epoxy, I will definitely go with the cheap stuff if yellowing is the only con, this thing wont be pretty anyway! That or just paint over the epoxy and then clear coat it.

I am planning on putting a lot of rocker in this board. I was going to base the middle section of board on my TT rocker and then turn up the nose by an extra 3/4" about 1/5 of the way down and similarly for the tail but only 1/8 ". I know this strays from the norm however my other skim is very close to Lonny's ideal measurements already. That being said it means I need a lot more rocker to abide by your ratio of rocker to concave so what about staying with 3x5mm sheets and just taking more material off the bottom. Would it be a mess if I went right into the middle sheet through the glue?

thanks again!

Olas

[tungsten]

Olas, I'll send you a design which works. "Lots of rocker on a skim" is a fruit bowl

I have made myself a skim for chop which works fine, you can check that and see if it appeals to you. Not too much concave needed on a skim either, maybe 2-3mm max, as a skim is quite wide and has little tail rocker. For waves and chop, you only lift the nose a bit, basically it's a tip kick. Basing a skim rocker on a TT would not work.

Shaping the concave is quite easy, because you see the layers in ply wood. PM me your email and I'll send you some stuff.

cheers, tungsten