Fiberglassing foam core kayak, Polyester vs Epoxy

Hay everyone, am new new to boat building and looking for advice. I was looking at getting a kayak for fishing and realized I had a lot of XPS foam board leftover from building our shop and after looking around found that it could be used as a core for kayaks and other larger boats. And since I have more time than money I figured why not build my own.

I am nearing completion of the sculpting and was looking to order fiberglass cloth and some kind of resin. I've been looking into the differences and generally understand that Epoxy is the better choice in a lot of ways for repairs and such but not a lot about fiberglassing an initial hull. I am aware that polyester resin will need a barrier between it and the foam as to not melt it. I have a lot of extra latex paint and can give it as many coats as it needs to act as a barrier if I go with that one. So any advice about which to chose would be appreciated.

Also if anyone knows what might work as a good putty for filling gaps between layers and tear outs in the foam that will work directly on the foam that would be appreciated as well.

 

How much does that lump of foam weight?

IMHO that is a terrible shape.
When you try to lay glass on those "strakes"??? you will need a big fillet to get the cloth to lay down.

I suggest epoxy. In part so that you don't have a weak layer (paint) between the foam and glass.

Styrofoam is the worst core there is.
What did you glue it together with?

Gap filler - epoxy with microballoons.

 

I'm pretty sure that if you try something other than epoxy it will disolve your foam. Test it on a scrap piece first. FYI. 2lb extruded foam has been used to make thousands of experimental aircraft, especially wings, stabilizers and rudders. Primarily because it can be accurately cut with a hot wire, great for airfoil shapes. But the strength comes from the layup schedule with layers of uni and bid, uni being the main contributor to strength. For thinner sections it's PVC foam, way better properties but more expensive too.

 

Welcome to the Forum Bren.
I would agree with Upchurch's comments above.
Have you done any calculations regarding buoyancy / floatation for this craft?
What are the approximate dimensions such as length overall, beam amidships and hull depth on the centreline? And how deep are the 'side hulls' below the flat middle section - I am guessing about 4"?
AND - reiterating Upchurch's question again, how much does all that weigh so far?
It will weigh a lot more by the time you have glassed it all over.
Will it then have enough buoyancy to support you and your fishing gear (and a few fish as well hopefully)?
And be reasonably stable as well?
Googling suggests that XPS board has a density in the range 28 - 45 kg / cubic metre.
1 cubic metre is approx 35 cubic feet, so approx 2 lbs / cubic foot as an average.

 

Actually, XPS can be glued with regular wood working glue - Titebond 2 has been what I used.
Since the foam is so weak it doesn't need much strength in the glue.

But I don't know of any way to laminate glass with Titebond.

As KiethO says, the foam has been used for aircraft, primarily as a tool creating the wing shape which is carried along with the aircraft. It can stabilize a skin a bit, but it isn't good as a core.

Boats such as you are making have been done before. Apparently they worked, but I have never seen a long term durability report.

Good luck.

 

The shape is loosely based off of the Bonefide SS127 which is designed for stability instead of speed. I've actually already tested mine, threw it into the water a few days ago in the state it is currently to test it out. Had no problems standing up in it even when another boats wakes. I didnt have a paddle with me when I tested so I dont know exactly how well it moves in the water yet but I'm not looking for something speedy. As for its current specifications its roughly 12' 5" long 33" wide and 9.75" from the current top to bottom, the "side hulls" are the deepest sections. Weighs 23 lbs and when in the water currently, with my weight (190 lbs) in it and no fiberglass yet, the bottom most point is only about 3.5" down in the water. The glue used between the layers was just the original gorilla glue thats supposedly waterproof and expands into a foam where it can. The foam is the pink XPS foam insulation you can buy at Home Depot and was used to insulate a 6" cement floor from the ground, dont have any exact specs on it but I know its waterproof closed cell. Again, I know polyester resin will eat this foam without a barrier, which I will have and test before actually using it if I went with it.

 

Beware.
Both poly and epoxy are thermogenic. Both will melt your foam if applied to thickly. A latex barrier doesn't provide any thermal protection. It will only prevent poly from dissolving the foam.

 

Blueknarr

I've put plenty of epoxy on XPS. Didn't melt it at all.

The problem with poly is the chemistry. Not the heat.

 

You could try "poor mans fiberglass" since the boat won't be in the water for extended periods and you have a lot of latex paint, here is one site. You'll have to round over any sharp corners to get cloth to wrap around them smoothly, plus sharp corners will get damaged and wear through easily.

Poor Mans Fiberglass, Permanent Exterior Paint, Never Paint Again (almost), Waterproofing With Out Epoxy, Fiberglass Epoxy Allergy Relief https://www.instructables.com/id/Poor-Mans-Fiberglass-make-nearly-anything-weatherp/

 

Epoxy for sure.
Do a little test piece first.
Don't forget your paddle this time!

 

Ya, I saw the poor man's fiberglass thing but every close up image of it I saw was really rough looking and you cant really sand it smooth so I'd rather do a fiberglass, sand then paint.

 

Hi Bren,

Welcome to the forum. That looks like Formular 250. Don't worry about epoxy melting it. I used the exact same product to build engine bearing stringers on a small cruiser I restored some years back. Used much heavier fiberglass over the Formular (which means much more resin than you will use on your boat) and had no issues whatsoever. Your biggest issues will be going around the outside corners and filleting the inside corners. Fiberglass cloth doesn't like a 90 degree turn. I'll see if I can find some older posts I did that might help you out as you look to cover your skiff. I don't have time right now.....

 

I have thermally melted XPS with epoxy fairing applied about a quarter inch thick trying to fill dings on the edges of a surfboard.

 

That is gonna be a bear to glass well. Ugh is all I can say.

Get your 36 grit paper out and start taking down all the sharp corners.

 

Ya, I expected the fiberglass wouldn't be able to lay out cleanly in one uniform sheet with this shape and was expecting to have to cut it in places and lay down extra strips to reinforce areas most likely to take hits. Was planning to use 6 oz fiberglass unless there's a reason to use a different thickness?