Poylester resin bonds

[Ross]

Because epoxy has greater bonding strength it is more forgiving of poor preparation. Stitch and glue boats are taped inside and outside. Therefore a peel test will not represent a true test of the strength of the assembly. I have a Bolger/Payson Nymph that I built 11 years ago the only fiberglass to wood bonds that have failed have done so because water got into the wood and it rotted. The boat was built with lauan plywood and polyester resin and I was careless about sealing the edges of the plywood.

 

[PeteK]

I have thought about using epoxy for the repair and new construction but I've been told that while epoxy will 'stick' to polyester resin quite well, the opposite isn't true. I was told by a 'glass guy to stay with what the boat was made of so as to avoid any problems in the future.

The work is replacement of rotten plywood interior bulkheads, "furniture", etc., with lots of tabbing.

I know now that I'll be doing lots of grinding to remove the old junk, and lots of dewaxing before attempting any bonding with any material. Oh, and the interior of the boat was painted sometime in its life with something, I don't know what. I'll have to remove that at least in the bottom foot or so and coat the 'bilge' with something like epoxy paint that won't peel off too soon.

If I hadn't obtained the boat for almost nothing I wouldn't be putting this work in to it!!

Thanks,
Pete

 

[Ross]

Pete, tabing is often done with epoxy and with good reason, it sticks better if the substrate is prepared well. The tabing I did with epoxy on my boat and the engine beds are set with epoxy. You can grind it off when it is in the way.

 

[Snotter]

Make all future repairs with epoxy and you won't have problems. If you need to save money buy something other than West. There are plenty of economical epoxies out there (i.e. Raca (sp?)).

 

[Ross]

Make all future repairs with epoxy and you won't have problems. If you need to save money buy something other than West. There are plenty of economical epoxies out there (i.e. Raca (sp?)).

One thing wrong with this statement, sometimes epoxy isn't the best choice. I can make a repair with polyester resin and put the repair in service in 8 hours or less. That can't be done with most epoxies.

 

[kenn]

I have thought about using epoxy for the repair and new construction but I've been told that while epoxy will 'stick' to polyester resin quite well, the opposite isn't true. I was told by a 'glass guy to stay with what the boat was made of so as to avoid any problems in the future.

The work is replacement of rotten plywood interior bulkheads, "furniture", etc., with lots of tabbing.

Any work where you're attaching to old fiberglass will be a mechanical bond, not a chemical bond... and epoxy excels at mechanical bonds. I also believe that it's easier for a novice to get good results using epoxy.

Last year I also had to replace a rotten wood bulkhead.After cutting out the rotten bulkhead, I used an angle-grinder and #80 sandpaper to roughen the bonding area, and cleaned up with acetone. I cut the new plywood bulkhead, checked the fit, then gave it one coat of epoxy+glass cloth (both sides) plus tabs. I then installed it in place, epoxying down the tabs, then one more layer of cloth + epoxy overall.

Note that any epoxy repair to the exterior NEEDS to be coated with gelcoat or a good paint, because most epoxy isn't UV stable.

Regarding polyester bonding to cured epoxy, it's been demonstrated that the mechanical bond of gelcoat to epoxy is more than strong enough. Here's one link, and here's one from West Systems.

 

[PeteK]

Great information guys! But a question Kenn: What about interior 'structural' bonding of polyester to epoxy, not gel coat?

And to make sure, you're saying that polyester-to-polyester would also be a mechanical bond, not chemical?

Thanks,
Pete

 

[Maine Sail]

And to make sure, you're saying that polyester-to-polyester would also be a mechanical bond, not chemical?

Thanks,
Pete

Pete,

Polyester to polyester is a chemical/primary bond if done before the original lamination has fully cured. Once the original structure has cured it becomes a secondary or adhesive bond.

 

[Ross]

Pete,

Polyester to polyester is a chemical/primary bond if done before the original lamination has fully cured. Once the original structure has cured it becomes a secondary or adhesive bond.

Secondary bonds are adhesive not mechanical. Duct tape does not form a mechanical bond, it creates an adhesive bond. Epoxy and polyester form an adhesive bond with the fiberglass and in new laminations a chemical or primary bond with the underlying plies. On old or repair work the bond between the new resin and the substrate is adhesive.

If you had a hole and ground a proper taper on both sides and then waxed the old surfaces you could still patch the hole by laying up the resin/glass laminate on both sides but there would be no adhesive bond to the old fiberglass. The patch would be impossible to remove because it was made with a primary bond across the hole and was bigger than the hole on each side. Thus you have a mechanical bond. It would leak of course. Threaded pipe is mechanically bonded or joined but the joints often leak so we use a sealing compound. Soldered pipe joints are metalurgically bonded and never leak if the bond is complete.

 

[Maine Sail]

Secondary bonds are adhesive not mechanical. Duct tape does not form a mechanical bond, it creates an adhesive bond. Epoxy and polyester form an adhesive bond with the fiberglass and in new laminations a chemical or primary bond with the underlying plies. On old or repair work the bond between the new resin and the substrate is adhesive.

If you had a hole and ground a proper taper on both sides and then waxed the old surfaces you could still patch the hole by laying up the resin/glass laminate on both sides but there would be no adhesive bond to the old fiberglass. The patch would be impossible to remove because it was made with a primary bond across the hole and was bigger than the hole on each side. Thus you have a mechanical bond. It would leak of course. Threaded pipe is mechanically bonded or joined but the joints often leak so we use a sealing compound. Soldered pipe joints are metalurgically bonded and never leak if the bond is complete.

Yep no coffee yet..Fixed my mis-speak...

 

[PeteK]

OK, got it. Since I have to replace all the interior bulkheading and such I've decided
to go ahead and use epoxy/glass for the job. The tabbing, wood-to-glass bonds, and new-to-old bonds will be much better than with polyester. Much better.

If this were a patch-up then I'd use polyester so as to not 'confuse' a future owner but
it's a full-on gut and re-do!

Thanks for all the info,
Pete