Using polyester resin instead of epoxy resin

So today I was removing some tabbing from a fir plywood bulkhead that had been done with polyester and csm only. I have to say I was impressed with how well it was bonded. Contrary to what many say and indeed what i have seen many times this was bonded as well as it would be possible to be regardless of resin type.The only way epoxy would have done a better job would be that you would have been using double bias so the glass would be better , not the resin system.On one side I ground it off with an angle grinder with a dust shroud and a zek disc but the dust pickup did not work very well as I discovered that there is a web down the center of the vacuum port (de walt) and pieces of glass wrapped around it and blocked the port, so modifications are in order. So the other side I removed most of the tabbing with a hammer and chisel to keep the dust down and finished with a sander. That tabbing fought me every inch, clearly if it ever failed in service it would be the wood fiber, not the resin system. On the other hand earlier in the week I did see some failed polyester bond in the bilge of a Beneteau First 42 I was working on sooo.

 

I too have seen polyester bond well, but often this is the exception, but the time the boat gets to me and needs repair. Most manufactures just don't put the time, materials and training into their laminators, so shoddy bonding is the norm in many cases. I could bond the crap out of plywood, using polyester, but I'd hate doing it and would always question if I should have used staples or other physical attachment, on high load areas.

 

Funny thing is this is on an old Gemini catamaran the i own and believe me they are about as poorly built as anything out there and yet all the tabbing is quite good, no sign of anything letting go anywhere and fairly neat although that's pretty easy with mat and poly. When I told my son about it I had to laugh as its the first time ive actually been impressed by something they had done. I personally don't use poly at all but do use a lot of VE and epoxy but I have used a lot of poly many years ago and do get a little annoyed at all the bashing it gets. As with most materials used in boatbuilding it is only as good as the guy using it. The bulkhead I am working on is actually more of a fore and aft structural partition separating the head from the fwd stateroom and I had to cut the tabbing up the front to slide some plywood through to stiffen the front due to poor construction and wanting to add hatches instead of windows so I have to re tab it which of course I will be using epoxy/ biax for.

 

Maybe your boat just had a good, experienced crew the day she got laid. I see this in older builds, especially the 60's, but not so much now.

 

Par, its hard to appreciate ( probably not the right word) the consistently low quality construction of these boats so I seriously doubt there was a good experienced crew anywhere near the plant, which is why I am impressed by the bond of the tabbing. I also have owned a Macgregor 36 cat in the past which was much better built, in fact better built than many of the much higher end brands that we have worked on over the years. All plywood to hull tabbing showed no signs of delam.

 

2 cents worth.
I’ve done lots of bonding of polyester to plywood in my laminator days.
First thing is plywood must be sanded, any means is better than none. Second scoring the area with a fibre sheet cutting tool after is even better. An X pattern repeated along the join.
A trick I was shown later on was to put a bit of accelerator (cobalt I believe) in some acetone and paint that on before laminating.
OP, if this is just a short term experiment polyester will do if you have some. If your going to buy resin then for the difference in cost buy epoxy and you’ll get some longevity for your money.
Make that 10c