polyurethane or epoxy

new to boat building have done wood working before an fell in love with it and couldn't help but to try boat building current project is a 4' halfpea for my little boy he is 5 and found one problem found that the epoxy resin is expensive and hard to get around here was wondering if i could sub it out for polyurethane but can't find a definite answer online i know it wouldn't last as well but will it work if so what are the problems that could crop up with stitch and glue boat building

oh for anyone wondering plans for the halfpea can be found at this website

http://koti.kapsi.fi/hvartial/dinghy44/dinghy4.htm

 

For stitch and tape for a small boat you can use polyester body filler for the filets and polyester resin for the glass taping, and of course decent alkyd house paint is just fine for painting like deck and porch enamel. You dont have to use epoxy.

Steve.

 

Having used both polyester and epoxy, I will always use epoxy from now on. No contest.

 

thank you very mush my son is hounding me to build his boat and didn't want to start till i knew if it would work just wondering what is the difference between polyester resin and fiberglass resin

 

Polyurethane glue is much more expensive than epoxy. There are several brands, but probably Gorilla Glue is the most popular. There are dozens of online stores that will ship epoxy to your door.

 

Polyester resin is usually what you see in a can marked "boat resin" or "fiberglass resin", but the label should be checked, which will tell you.

I disagree in that polyester resin and Bondo are suitable for taped seam builds. Neither Bondo, nor polyester stick to wood very well.

Epoxy is easy to get on line, drop shipped to your door, usually less than half the price you'll see at West Marine or the local marina supply store.

Try Progressive Epoxy (>www.epoxyproducts.com<) or E-Boat (>www.bateau.com<) as both have epoxies at much lower pricing than the typical retail stuff you're seeing.

Gonzo is correct, polyurethanes are more costly than epoxy and can only be used as an adhesive, not a coating, such as epoxy. You can't stick fabrics down with polyurethanes either.

 

I just knew that everyone would disagree with the polyester thing but in fact its a reasonable solution if you dont wish to spend the money for epoxy. OF COURSE epoxy is better but polyester works, in fact by far the majority of older stick built production boats sailing the planet have their plywood parts tabbed in with the same polyester resin that their hulls are built with. 30 years ago we developed a range of 13 different stitch and tape boats for a company named Country Ways, these kits were liscenced from a british company named Bell Woodworking, most were designed by Jack Holt. We recieved one kit of each model and had to refine them for efficient production, the British kits all came with polyester resin (we supplied the ones we built with Epoxy)
Polyester bodyfiller sticks quite well to plywood actually and will usually pull the veneer apart if you try to separate the pieces and you dont need better than that. I Worked for a few years as a patternmaker at a large foundry and we used quite a few cool materials such as castable urethanes of different densities and used lots of polyester fillers that were much like bodyfiller but formulated for the pattern industry, one that was really cool was called Tuff Carve which as the name suggests never got rock hard but remained carvable like wood.

Steve.

 

from personal experience and that of my customers, the polyester bond to wood fails after 5 - 10 years (the reason fiberglass boats are 100% fiberglass). Polyester resin also has unhealthy fumes and pot life is rather short and unpredictable depending upon the age and amount of catalyst. Epoxies are generally odorless, bond better etc.

paul oman
progressive epoxy polymers

 

Yep,epoxy is better for sure, no doubt about it, but we are talking a little dinghy here where the Op has expressed a aversion to the cost of epoxy. Polyester does not necessarily fail after 5 - 10 years but it may, there are many thousands of production boats plying the oceans of the world with all their major structural components tabbed in with polyester that havnt failed, as with all goo sucess depends on proper prep and workmanship. Weve all seen failiures of polyester to wood interfaces but it usually comes from poor work practices, ive used thousnds of gallons of epoxy over close to 40 years but its not the only viable choice for every situation. Actually,the pot life is easily controlable, by varying the catalyst ratio, something you cant do with epoxy as you know, you have to buy another hardner further adding to the cost, and of course mixing smaller quantities at a time and pouring off into a shallow tray are tried and true methods for polyester or epoxy. Again, Epoxy better,polyester more than adequate for this job if cost is a concern.

Steve.

 

I think we might have lost mrjcwhite in the discussion.

As an amateur mrj, I'd strongly suggest that you use epoxy. PAR is right about finding it online. Retail is waaaay to expensive. Once you get used to using epoxy, you'll never go back to poly. I even use epoxy to do auto body work on my 1996 Suburban. People often ask how I keep it looking so good. Epoxy really does create a better water barrier than polyester. You won't be saving much using poly because your project is small. Go with epoxy.

 

Yes, you can make polyester work Steve, but for the back yard guy, epoxy is a much easier goo to work with. The cost differences are slight if you actually count everything up in the end and you'll have a stronger, more waterproof result too. Learning how much or how little catalyst for the day's environmental conditions using polyester, can be daunting enough for the novice laminator, let alone good techniques, appropriate materials and layup. Certainly epoxy has it's quirks too, but you can screw up pretty good and still get a product better then polyester. As to the cost of epoxy verses polyester, well this really isn't as big an issue as suggested. An epoxy laminate will need less resin and less material, for the same strength and stiffness joint than polyester, so you save in goo and materials, offsetting to a degree the price difference for the resin. If the epoxy is not one of the major brands, you can rival the cost of good polyester, so the price differences aren't as significant as they once were.

 

If you cant use epoxy then dont build stitch and glue !

Pick a ply on frame Micro pram or a simple Mini Dory to build.,

 

ok thank you

 

Keep focused on the boat that the OP plans to build, thats what were talking about here, and that is what im suggesting polyester as being adequate for. There will be no savings in material with an epoxy laminate, he will be wiring it together, applying a filet on the inside, one layer of 2" glass tape inside and out, thats regardless of what resin you chose. It will last 20yrs with poly and be just as "strong" what more do you want with a boat like this? Dont get me wrong, im not advocating polyester as a substitute for epoxy for every job, but perfectly adequate for many. I agree that staying away from the major epoxy brands will save you some bucks but on a little job like this you cant beat the availability of polyester, in fact you can buy almost all the materials you need for a project like this at a big box lumberyard, plywood, copper wire, bondo, polyester resin, glass cloth, (probably not glass tape) solid lumber, and some good old porch enamel, no pumps, powders or shipping to pay for, it will be a lot cheaper on a small boat like this.

Steve.

 

I'm with Par - the few pennies ( if any ) saved on Polyester is just not worth the hassle. This boat is so small that the amount of goo wont be any financial significance. Why he would want lumber from the h'ware store, I have no idea - this is an all ply plan.

Poly may survive with love and care, but who needs the worry.

I have seen enough delaminating Poly ply dinghies in my lifetime, as the moisture gets behind the FG.