Question about resin

[twistedwrench78]

I am getting ready to replace the transom in my boat. I want to use epoxy for this since it’s supposed to be stronger than poly. I believe I will have some Epoxy left over and plan to use it for filets on the stringers as well. Once I run out I would switch to the cheaper poly for the rest of the stringers and deck work.

If I use epoxy for the filets on the stringers and let that cure, can I do the tabbing and capping with poly? Or will there be any issues getting the poly and fiberglass to bond with the cured epoxy filets?

Never done something like this before but I researched on here to kinda get the basics and watched Jminks glastron build so I feel like I can get through this once I start.

Thanks in advance.

 

[tpenfield]

Poly does not readily adhere to epoxy, because of something known as 'amine blush', which a waxy by-product of epoxy that resides on the surface as it cures.

Poly is plenty strong, just more brittle than epoxy. Epoxy has very good adhesion qualities . . . a bit different than strength. If you start with epoxy, you may end having to use epoxy throughout your re-build. The cost of doing such can get a little scary.

There is Vinyl Ester resin, which is similar to poly, but has better adhesion, like epoxy. It's working time tends to be longer than poly.

Poly resin is fine for most re-builds. I use VE resin most often since it is a good step up from regular polyester resin.

 

[chevymaher]

LOL watch boatworks epoxy is more brittle and broke in the test before poly. Poly don't stick to epoxy. Gel coat don't stick to epoxy. Once cured epoxy don't stick to epoxy. It is batting a thousand I see where it gets its reputation.

It is for a quick fix sell and good luck in the future deals.

 

[todhunter]

I would just stick with polyester. Before I did my boat, I heard all the "negatives" for polyester and "positives" for epoxy, but after working with it, I think polyester has way more positives than the internet gives it credit for.

 

[Scott Danforth]

Tried Epoxy, wont go back to epoxy

Unless you are using carbon fiber which requires epoxy, i would stick with poly

 

[racerone]

You would wonder how they build modern aircraft using carbon / epoxy composites.-----I will stick with epoxy as there is virtually no smell and worth the extra coins just for that benefit.

 

[Scott Danforth]

You would wonder how they build modern aircraft using carbon / epoxy composites.-----I will stick with epoxy as there is virtually no smell and worth the extra coins just for that benefit.

pre-preg and a big oven

 

[76SeaRay]

I used epoxy for the structural work on my intermediate hardtop (the one between the upper and lower windshields in my profile pic) and for ribs in my upper hardtop for the strength. That wood (marine plywood) will all be covered with headliner so I don't have to "stick" anything else to it. The rest of my restoration (transom, stringers, and floor) uses poly due to the expense of epoxy and the difficulty in "wetting out" the cloth with epoxy compared to poly.

 

[tpenfield]

I use epoxy in situations where adhesion is important. Mostly VE resin in projects and repairs. It seems to have a longer working time than poly and it tastes better.

 

[Scott Danforth]

it tastes better.

You ate paste as a kid.....didn't you?

 

[KJM]

Poly is what most boats are made from and is plenty strong. Considering thats likely whats already in your boat and thats what you plan to finish with it makes more sense to just use it for all your needs.

 

[twistedwrench78]

Thanks for all of the replies. Luckily I didn’t buy to much epoxy, I’ll use it for little projects around the house/shop. Time to order some poly.

 

[DeepBlue2010]

I see you got some good advice from top experts on the subject. But, for the benefit of newcomers, this question has been one of the most FAQ on the site so I will try to sum up what was said before.

Once epoxy always epoxy. Any poly products (including Gel Coat) will not adhere to Epoxy underneath. A simple mnemonic device to help you remember this fact is Epoxy is always on top of poly (price-wise, of course)

Good prep work including - but limited to - pre saturating the wood before lamination is a key to success with both resins. A very nicely prepped Poly job may end up stronger than a slopy Epoxy one.

Resins on their own are of little value as far as a final product is concerned.
Expoxy can be used as glue on its own but I am talking about a laminated part. The entire laminate's components (fabrics, resin, and schedule (i.e. which fabric goes first, 2nd, 3rd, ......., etc)) are what make the weight to strength ratio of the final laminate and determine its quality for its design goals.

Only compare apples to apples and oranges to oranges. Airplanes are different animals where weight-to-strength ratio differences no matter how small, are of paramount importance to the design's success. While in boats, it could translate to near-zero ROI. Now, if you are building a speed boat to race with, that is a different story. Match your design goals with the right materials for it otherwise, you will be overbuilding which is OK if this is what you want to do and money is no issue. But OK doesn't necessarily translate to "better". In short, a boat that was built entirely out of Poly will probably be satisfied with a Poly repair job.

Finally, did I stress enough the importance of prepping?

Best of luck. Stay safe!

 

[Chris1956]

If epoxy is sanded, poly will stick to it, just fine. I have never had an issue mixing the two, as long as both are fully cured, sanded and wiped with acetone.
As was stated, poly is fine for most repairs. For example, for added strength on a crack in the gunwale of my speedboat, I used epoxy. I feathered the crack on the outside, and laid oval shaped glass cloth pieces to refill the original thickness, and used a double layer of heavy roving on the inside. These were saturated with epoxy in one batch, and allowed to cure. The outside was sanded, wiped and faired with poly putty. After cure it was sanded and painted. No issues more than a decade, and much hard use.

 

[Fiberglass 1 inc.]

If epoxy is sanded, poly will stick to it, just fine. I have never had an issue mixing the two, as long as both are fully cured, sanded and wiped with acetone.
As was stated, poly is fine for most repairs. For example, for added strength on a crack in the gunwale of my speedboat, I used epoxy. I feathered the crack on the outside, and laid oval shaped glass cloth pieces to refill the original thickness, and used a double layer of heavy roving on the inside. These were saturated with epoxy in one batch, and allowed to cure. The outside was sanded, wiped and faired with poly putty. After cure it was sanded and painted. No issues more than a decade, and much hard use.

Most, but not all, epoxies produce an amine blush on cure. This amine blush is removed with water. Acetone and alcohol only smear the blush, water is the only solvent for amines and should be scrubbed with a scrub pad and lots of water. Just yesterday I made the mistake of assuming that a heavy rain on an epoxy repair would have removed the blush. It didn't and I had to remove the gel coat that was applied and scrub the epoxy heavily with water. The next gel coat application was fine.
This is the only time in 35 years of gel coat over epoxy that I ever had a problem. We only do this with WEST epoxy repairs. There are many epoxies that absolutely should not be overcoated with polyester products.

 

[zool]

If youre worried about a poly product sticking to epoxy, just use a layer of vinylester as a tie coat. It works fine. You still have to wash off the blush tho with water.