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How to make a cheap stitch and glue
Bit of background. I made three stitch and glues about 10 years ago. In my normal way, I started out building expensive but ended up building inexpensive. All three boats have revealed no faults and all look now pretty much like new - none of the incrementally cheapening decisions I made about materials made the slightest difference - at least not in the first decade of their lifespan.
Wooden canoes will last a very long time if they are not left to sit in water, not left with water sitting in them, and kept out of the weather and sun. When done, I take my boats out of the water, dry off any obvious water, and store them in the garage. If your kids wont do that, or they are hard on things generally, save yourself the pain and buy a cheap secondhand fibreglass canoe.
But if you want to go ahead, and to limit the spending, this is what I'd use.
Consider using a good quality outdoor ply. Marine plies are better, no question of that, but outdoor plies use the same waterproof glue. The plies are thicker, there will be more surface faults, and there may be internal voids if you don't choose well, but a well chosen external grade ply will do the job for kids. I have a kayak a friend made of external ply when we were kids (marine ply, fibreglass and marine paints were far too dear for us back then) and its still with me and doing well today - 45 years later. Well built and always treated well. Shop around and find external ply with no voids - it is possible.
For marine ply I used variously Okume (expensive) Fijian cedar (medium) and cheap nameless Asian-sourced hardwood. All have done the job well, though I think the Okume was nicest because it was lightest and strongest.
If you go marine ply, the Bunnings stuff is the same as the Asian sourced hardwood mentioned above. It will be fine though it will be a bit heavy and stiff. At least you wont be destroying any desperately endangered West African rainforests.
Use epoxy as a glue only - ie use it for scarfing, taping seams, filleting seams, gluing on gunwhales etc. I think you will need 1-2 kg for that.
Use polyester resin for sheathing the bottom, and maybe inside around the cockpit as kids are pretty clueless when getting in and out.
Any fibreglass cloth will do - its simply not a quality issue.
Use a marine varnish for topsides. Skip the epoxy soak coat. A full epoxy envelope is a wonderful thing to have, but its expensive and more suited to something you want to last forever.
For the few timbers that are required, I used recycled Oregon from our old pergola. A light wood perfect for ribs, gunwhales, bulkheads, keel or whatever else is needed.
In one case I needed those frames that run perpendicular to the keel. These were 19mm thick or thereabouts. I used ordinary external ply for these. There were voids. I filled them. It hasn't mattered.
For paint, I love marine paints but they are expensive. Basically, the difference between marine paints and house paints is one of incremental quality and UV resistance. A house paint will do the job if you store the boat as mentioned above.
I think if you do this you will get a boat with a 10-20 year lifespan if treated well.
Id probably still be going secondhand fibreglass for kids.
If I can find it, I'll attach a photo of one of my stitch and glues, for motivation.
cheers
Arron