Polyester vs Epoxy

[blxm]

Just been watching the Videos from ‘Followtheboat’ https://www.youtube.com/user/spannerheadz/videos and it struck me that the vast majority of GRP projects, in other channels and threads, seem to be using epoxy. I can understand the use where strength and adhesion to existing structures needs to be as high as possible and epoxy does have advantages here, although the few 'experiments' on adhesion to existing GRP that seem to have been done seem to suggest that this is not as critical as we might perceive.
I've used both over the years in non marine environments making low volume items for race cars and most volume boats I gather are constructed with polyester type resin.
So the question is, given the cost advantage and ease of use that polyester has over epoxy why is polyester not used more often? Have I missed something?


PS I'm not suggesting that epoxy should never be used, there will be instances where this is clearly the appropriate resin.

 

[coopec]

In Australia I've noticed Aluminum has pretty well much taken over from fiberglass
Ride Comfort – Fibreglass
Toughness – Aluminium
Price – Aluminium
Reparability – Aluminium
Customizability – Aluminium
Maintenance – Aluminium
Aesthetics – Fibreglass

http://bluehq.com.au/aluminium-vs-fibreglass/

I don't have hard facts at hand but I would be very surprised if there were any remaining fiberglass boat manufacturers who would use epoxy. Compared to aluminum they wouldn't be in the race.

 

[LadyInBed]

Are we talking sticking bits onto the boat or constructing boats?

 

[PuffTheMagicDragon]

Most probably it is a question of marketing. For marine applications in wood, especially laminations, I used to prefer resorcinol (Cascophen) and it never failed me. This is now not available any more in Malta because people automatically ask for epoxy. As a result I now get resorcinol adhesive directly from Germany.

 

[lw395]

Lots of racing dinghies are built from epoxy. Lots more are built of vinylester and more still of polyester.
Epoxy has advantages for some repairs and for gluing on other materials.
It also comes into its own when using carbon or kevlar in the layup.
For small repairs the price premium of epoxy does not matter.
For cosmetic repairs, it can be better to repair polyester with polyester. Gelcoat applied to epoxy is sometimes troublesome.
A lot of people have done or seen bodged repairs with polyester, and seem to think it's hard to use to achieve a proper repair with the integrity of the original hull.
There is some skill to doing it properly.

 

[blxm]

Are we talking sticking bits onto the boat or constructing boats?

Mainly making parts for projects or making new parts. For example the "dodger" (as they call it) in the videos I linked to. Also recently came across a video of a chap making a replacement locker lid another making a pedestal for a motor boat and another making a solid Bimini. All in epoxy. Maybe a bit of confirmation bias
There is also saillife where Mads re-cored the deck and used epoxy to glass over the new core. Would polyester layup have been OK?

 

[lw395]

Mainly making parts for projects or making new parts. For example the "dodger" (as they call it) in the videos I linked to. Also recently came across a video of a chap making a replacement locker lid another making a pedestal for a motor boat and another making a solid Bimini. All in epoxy. Maybe a bit of confirmation bias
There is also saillife where Mads re-cored the deck and used epoxy to glass over the new core. Would polyester layup have been OK?

I would say epoxy is pretty much wasted on those projects. But if you have epoxy in your shed, and you are familiar with using it, then maybe there is a case for sticking (pun!) with what you know? Materials costs are not everything.
Epoxy needs protection from sunlight, which in the long run may mean maintaining a paint finish.

 

[coopec]

I'd agree with you: if you are fabricating hatch lids, dorade boxes etc polyester is the way to go. I would use epoxy if I was repairing f/g or attaching something to polyester resin lay-up. I thicken up epoxy resin with talc to make up an epoxy glue.

 

[Keith 66]

I have spent 35 years in the boat trade & did a lot of repairs, some huge.
Good GRP repair is largely down to preparation, making sure that the damage is ground out properly & well feathered in with long tapered edges. If this is done well polyester will be fine & nobody will tell the difference.
Many insist on epoxy, true it is a better adhesive but is it compatible with the binder on your glass reinforcement? If not you would have been better using polyester! Another option is Vinylester, ease of use of polyester but performance approaching epoxy.

 

[Akestor]

Before 3 months i patched 8 hull holes bellow the waterline on my Westerly. I was wondering if i should use polyester or epoxy resin. Finally after research and common sence i went with polyester because i wanted the repair to be as close possible to the original fiberglass . If i used epoxy, i would have made these areas much stronger and stiff and probably the bending of the hull during rough weather would not be uniform. Between local strength and uniformity i choosed the later! I ve never heard of a polyester patch unstick from the original hull.

 

[William_H]

Before 3 months i patched 8 hull holes bellow the waterline on my Westerly. I was wondering if i should use polyester or epoxy resin. Finally after research and common sence i went with polyester because i wanted the repair to be as close possible to the original fiberglass . If i used epoxy, i would have made these areas much stronger and stiff and probably the bending of the hull during rough weather would not be uniform. Between local strength and uniformity i choosed the later! I ve never heard of a polyester patch unstick from the original hull.

I don't know of any difference in the stiffness of the final GRP between epoxy and polyester. The stiffness being largely dependant on the glass or other filler material. So I don't think (or never heard of) epoxy being bad in a repair on polyester boat.
I do know that while polyester is the material of the original GRP it is not the best for repair. ie replacement of material. This is because polyester is cheap and sticks wonderfully to polyester which has not cured. (as in cured for weeks) It will gain a chemical bond to various degrees if the bottom layer is hard but not old. ie next day. But old polyester is a solid material that can only be adhered to by mechanical means ie new resin runs into micro roughness and grips.
Epoxy does do the mechanical gripping better plus has much less shrinkage when hardening than polyester.
However as OP said polyester as a repair "glue" can work OK with large areas of contact and perfect cleanliness before application.
As said vinylester resin is really good stuff many advantages of epoxy like good adhesion but is more stable in UV light and cheaper than epoxy plus add more hardener for quicker hardening in cold. Not so easy to find however.

 

[Akestor]

I don't know of any difference in the stiffness of the final GRP between epoxy and polyester. The stiffness being largely dependant on the glass or other filler material. So I don't think (or never heard of) epoxy being bad in a repair on polyester boat.
I do know that while polyester is the material of the original GRP it is not the best for repair. ie replacement of material. This is because polyester is cheap and sticks wonderfully to polyester which has not cured. (as in cured for weeks) It will gain a chemical bond to various degrees if the bottom layer is hard but not old. ie next day. But old polyester is a solid material that can only be adhered to by mechanical means ie new resin runs into micro roughness and grips.
Epoxy does do the mechanical gripping better plus has much less shrinkage when hardening than polyester.
However as OP said polyester as a repair "glue" can work OK with large areas of contact and perfect cleanliness before application.
As said vinylester resin is really good stuff many advantages of epoxy like good adhesion but is more stable in UV light and cheaper than epoxy plus add more hardener for quicker hardening in cold. Not so easy to find however.

Thanks for mentioning the qualities of the resins. Just in case it might be useful , I helped the laminate get a chemical bond by first making a flat internal patch that its center ( original hole) would get a bond with the external , the next days. As for the mechanical bond, i beveled the areas with 36 grip, blow the dust and cleaned very well with acetone. I was aware of the shrinkage but i had to live with that, time will tell i guess. Another disadvantage of polyester is that its not the best for underwater use ( vinylester is better and epoxy much waterproof). After i sanded
the patches i applied epoxy primer for sealing, then normal primer and antifouling.

If i was making the repair again, i would first apply thickened epoxy to the beveled area and right after, (BEFORE the epoxy cures) i would start laminating with normal polyester and the glass of choice. This way the whole patch would have a better adhension, and would keep almost the same flex properties of the original hull.

ps-I am not an expert or proffesional, just a DIY that enjoyes to reffit/repair everything onboard!

 

[William_H]

If i was making the repair again, i would first apply thickened epoxy to the beveled area and right after, (BEFORE the epoxy cures) i would start laminating with normal polyester and the glass of choice. This way the whole patch would have a better adhension, and would keep almost the same flex properties of the original hull.

ps-I am not an expert or proffesional, just a DIY that enjoyes to reffit/repair everything onboard!

I must say the idea of applying polyester resin to uncured epoxy makes me shudder. You won't get a chemical bond being different kinds of long chain polymers. Indedd you may get an amine blush from the epoxy which will hinder adhesion. I am not sure about this however. I would stick with epoxy or polyester. As I said I don't think the flex properties of the 2 resins is a concern. If you can get a patch on the inside to match the patch on the outside with suitable bevelling on both sides the Layup from outside can chemically bond with that on the inside producing a plug which will stay in place regardless of adhesion. So polyester would be fine. olewill

 

[Akestor]

I must say the idea of applying polyester resin to uncured epoxy makes me shudder. You won't get a chemical bond being different kinds of long chain polymers. Indedd you may get an amine blush from the epoxy which will hinder adhesion. I am not sure about this however. I would stick with epoxy or polyester. As I said I don't think the flex properties of the 2 resins is a concern. If you can get a patch on the inside to match the patch on the outside with suitable bevelling on both sides the Layup from outside can chemically bond with that on the inside producing a plug which will stay in place regardless of adhesion. So polyester would be fine. olewill

I have tried this recipie inside the boat reinforcing the floor connection to the bildge where it was unstuck. The idea is that once you apply the polyester laminate over uncured thickend epoxy, the polyester laminate cures first, and the epoxy is still uncured. So it is the same as you would glue 2 cured polyester laminates with epoxy. Applying polyester over cured epoxy would be a mistake ofcourse ( haven't tested this myself but trust what i ve read on internet about it).
Thanks for the constructive feedback