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Can Polyester Resin be used to join plywood for a boat?

While 'Stitch & Glue' construction is commonly associated with the use of epoxy resin along with an appropriate filler, even polyester resin and polyester auto-body filler can be used in a similar way for a low cost project.    Although you’ve probably seen website warnings to ‘never use polyester as it bonds very poorly to wood’, my own tests and the building of 3 successful boats using polyester between 1960 and 1980, satisfied me that there are ways to make it work.  Here is what I discovered.  

Coved resin joints that cover or replace the initial wires, are commonly covered with a cloth tape but when this is used with polyester resin, it sets up very rigid and does delaminate rather easily … particular if used against a waxy wood or a ply with a surface veneer of Fir or Teak.   But ideally, to lay in a corner cove, the glass fibers need to lay at an angle to the coving, such as is achieved when you use a 45/45 biaxial.   But as polyester is more commonly used for an economic project, this is where the use of a tape cut from CSM or ’Chopped Strand Matt’, is very effective.   This material can be easily separated into thinner layers but the random spread of the 2” long fibers are perfect for a tape to lay over a polyester coving.    The first boat I used this on was a 2-man racing sailboat (I called ‘Flying Gnat’) built 10 years before epoxy was readily available, on which I first tried my version of stitch & glue, but using automobile polyester.  [The Gougeons first played with epoxy for boats in the early ‘60’s but their WEST business only started about 10 years later].   The hull skin was only 3mm mahogany ply, but I stiffened it with a layer of glass, also using polyester with a color added in, so it was like a gel coat.   The surface was very hard and I made sure no plywood or sheathing edges were left exposed.  I did thin it with about 10% acetone if I remember correctly, as it tends to be rather thick.    (Something you should not do with epoxy for any strength joint).

My next use was in 1973 on an 11ft dinghy design I created to build with my 13 year old daughter.   We named the design ’Mosquito’ so her boat became known at 'The Sting' ;)    Here we used 3mm birch ply with all joints made of polyester auto-filler covered with CSM tapes wetted in polyester resin, lightly thinned with acetone, with 3/4" polyurethane foam board then bonded to the interior (with WP contact cement) to add essential stiffness. 

The 3rd and final test was in 1979 on a 15ft double-chine canoe design I called ‘MicMac’.   This was another inexpensive 'family project' to try to interest my 12 year old son in boats and boatbuilding.   (Bad education planning though, as I'd just bought him a small 2nd-hand Honda moto-cross to re-build, and that was way more like the Meccano Sets he was graduating from :).  

This canoe used the same system as the Mosquito, (a system I was then calling Plyfoam*),  but this time I put a single layer of 6oz fiberglass over the paper that sheathed the PU foam, which better controlled bilgewater from getting through to the thin hull shell ... something that rotted the Mosquito ply in just 3 seasons .. aided on by numerous capsizes as the girls had a ball.   But that glass was still laid in with polyester resin for the MicMac.  (Note that the main carrying beam was my sons once-favourite hockey stick ;)

  

That canoe was used a fair amount, stored upside down outside and lasted 28 years (!), so by then, I obviously had the use of polyester well under control.

When I finally had to cut the canoe up, most of the painted 3mm birch door skins had totally rotted through, except in way of all the chines where the polyester and CSM was still 100% effective and there was not a single inch of the 80+ft of joints showing any sign of delamination or cracking away from the plywood, which was still very sound under the joint material.    Here (bottom right) is a photo of the joint after 28 seasons of service, while the surrounding (non-marine) ply is now trash.

So when you combine polyester filling with corner tapes cut from 2oz CSM, this makes a rigid but perfectly sound joint with relatively inexpensive material.    It IS heavier than when using epoxy though and of course as we know, has a strong, obnoxious smell so preferably needs to be used outside. 

Some will rightfully point out that CSM is just not very strong.  But as the bond to the wood is so much better, the overall joint proved to be totally adequate and over a significant time period.  While regular woven tape is theoretically stronger, it does (when used with the stiffer polyester) delaminate far more easily, so I would not recommend this with polyester resin.   Another convenience of using CSM fibers is that the binder used to temporarily hold the random glass fibers together, is typically designed to break down more readily with polyester than with epoxy.

NOTE: Interestingly, all these 3 boats had hulls made of just 3mm plywood.  (The first one was marine ply, but not the other two.  Because of this, it may be worth considering that part of the success in these applications could be due to the relative flexibility of the adjacent plywood, so using polyester with thicker, less flexible plies say over 8mm, would require further testing before concluding that the joint was 'automatically' adequate.  Tapering the coving edges and using sufficient glass would certainly help.


  *Plyfoam.    Although not designed for rugged, long-term use, this inexpensive system proved to have some interesting positives.    By bonding a semi-rigid closed-cell foam to a plywood that initially was clearly too thin and flexible to take much weight, the plywood then became the tension face of a composite panel and was remarkably changed in performance.   Even the 4.25' beam Mosquito could then take 2 adults without visual deformation.    I used the same simple combination for the thwarts, with a layer of this paper-backed PU foam bonded over a base of 3 mm birch plywood over a simple frame of two 19 x 19 pine strips, hanging on 3mm ply brackets either side.  It was totally strong enough and the foam proved to be both warm and very comfortable as it slowly deformed to take the shape of a human buttock ;)   The paper over the foam was painted with two coats of regular Varathane paint and was still great after 28 years on the canoe.   Over the life of these two boats, there was never any failure of this crude composite combination, other than a few chips broken out of the foam.   Hardly what one might call 'beautiful' ... but comfortable, warm, lightweight and surprisingly durable for a very inexpensive boat.

PS:  In case anyone is interested, 4 sheets of detailed drawings were created for the Mosquito after the first one was built (dated 1975) and as these were recently scanned, they are now available by email for US$75.  For the MicMac canoe, no plans are really needed once the hull building frames are created from rough lumber.    A cross section and table of offsets are available though (US$35).    The boat has an efficient, easily driven double chine hull with fairly full ends for an efficient prismatic coefficient. The topside tumble-home and interior gunwales (inwales) also made for easy paddling.   Best canoe I've owned.  Weighed about 42 lbs when new.   While the raised ends were true to tradition (giving a boat to sleep under when 'capsized' on the beach), we found they created annoying windage on an open lake, so we cut them down after 2 seasons to a more normal sheer line.      If the offsets were entered into a program such as Delftship, the expanded shape for the 6 plywood strakes would be available.   If anyone is interested, they can have the offsets free in exchange for a copy of the shell expansion.


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