The great debate -- epoxy vs poly layups

[Dabbler_E]

Lurking for quite a while, and now gearing up for the obligatory transom, stringers, & deck job on a 79 Terry 450 cm, so I'm considering the great epoxy vs. poly debate

The arguments I've seen seem to boil down to:

Poly: cheaper, more than good enough given good prep, fast work time

Epoxy: stronger bonding, more forgiving, relaxed work time

What I haven't seen much of are hard numbers about how much less epoxy one would use for the same job.

Looking at the fiberglass info sticky, I see that each layer of 1.5 oz csm uses about as much resin as a layer of 1708 or 18 oz roving.

I've also read that csm is really only needed as an underlayment for cloth when using poly (not for exposy), to improve the bond to the underlying substrate & because the poly resin needs more help to be strong in multiple directions than epoxy does. If so, wouldn't that cut the resin schedule nearly in half, leading to a much more even resin budget with the two materials? Also, wouldn't epoxy allow a lighter weight roving or cloth to be used?

Hunters use conversion tables for lead vs. steel vs. tungsten etc. shot size equivalents giving similar impact down range -- it'd be great to have a similar table for epoxy vs. poly layups (e.g., what layup w/ epoxy would yield equivalent strength as a layer of 1708 w/ poly, assuming good (but not amazing) quality resins and proper prep?)

So, poly and epoxy gurus:

As an example, what would be your respective layup schedules for a 5" tall stringer on a 15' (450 cm) long, relatively narrow bass boat?

Let the games (hopefully not flames) begin!

 

[Robert4Winns]

Re: The great debate -- epoxy vs poly layups

I'm no expert, but since you asked about how much less epoxy you could use as opposed to poly, the way to save on the amount of epoxy is to use 1700 as opposed to 1708. Poly needs the mat, but epoxy does not. I think some of the on-line fiberglass suppliers have the information as to how much resin is required to wet out the different materials.

 

[Dabbler_E]

Re: The great debate -- epoxy vs poly layups

That's the sort of thing I'm looking for R4W -- I've looked up how much resin different materials require. What I'm looking for is the difference in materials one would use w/ epoxy vs. poly.

 

[jonesg]

Re: The great debate -- epoxy vs poly layups

Poly wets out so easily you could end up using more epoxy in the end.

I don't see any debate, its your money, buy what you want to.

 

[ondarvr]

Re: The great debate -- epoxy vs poly layups

In general epoxy can be about 1/3 stronger, but there are so many formulations and types of both products that this is just a guess unless you actually know the physical properties for the exact resins you plan to compare or use. Then, typically, the physical properties of epoxies are listed for the product after its been post cured (put in a large oven and heated at X temp for X amount of time), which can be much higher than ambient cured resin.

The other thing that frequently happens is the person uses about the same amount of glass with either resin, so it ends up about the same volume of resin.

No mat is needed with epoxy, so that helps, but good epoxies tend to be rather thick and don't wet the glass as well polyesters, plus they tend to sag, the slow cure sometimes compounds the sag issue .


If it cost less, or was close in cost to use epoxy in building boats (or anything) many of them would be made with it. The problem is in the real world the cost of using epoxy is much higher than just the price per gallon when compared to polyester.


There is no official scale or guide to laminate strength as there is with metal, there are just too many combinations of resin types, fiber styles, resin content, production methods, etc.

In the end it just comes down to using the product you want to use for whatever reasons or factors seem more important to you. They will both work well enough that the repairs will last much longer than your desire for the boat.

 

[rucaradio]

Re: The great debate -- epoxy vs poly layups

Bottom line is IT'S UP TO YOU!

This debate is never ending. When I did my restore last year I opted to use poly. I figured the boat was held together for 21 years with it, and for the price and amount of resin needed it would've been foolish for me to use epoxy.

If you read up on poly and mix the MKEP properly it won't be a problem. I bought my first 2 gallons of resin and roll of glass from US Composites, but then I found a local shop which manufactures fiberglass components for aircraft that sold me the resin by the gallon CHEAP. Not many "local" fiberglassers will stock epoxy resin - it simply isn't cost effective.

I figured if poly could hold together a plane it could hold together my hull..

 

[Dabbler_E]

Re: The great debate -- epoxy vs poly layups

Bottom line is IT'S UP TO YOU!

The thing is, I'm not looking for suggestions on which to use, I'm looking for information upon which to base my decision.

Again, for those with lots of experience with either material:

If you were to glass in a 5" tall stringer in a 15' boat, what layup schedule would you use for poly? (e.g., 1.5 oz csm then 1 layer 1708, or just the 1708, or 10 oz cloth between 1.5 oz csm?) For epoxy (e.g., just 1700 biax alone? Just cloth alone? what weight?)?

I'm not looking for a boat that will survive a torpedo strike, just a solid reliable hull.

Thanks!

 

[ondarvr]

Re: The great debate -- epoxy vs poly layups

quote

"If you were to glass in a 5" tall stringer in a 15' boat, what layup schedule would you use for poly? (e.g., 1.5 oz csm then 1 layer 1708, or just the 1708, or 10 oz cloth between 1.5 oz csm?) For epoxy (e.g., just 1700 biax alone? Just cloth alone? what weight?)?"
quote





Its back to the "too many different designs" there are 15' boats that don't need or have stringers, then are models with several. Sometimes the only reason for them is to support the floor. Its hard to say on a 30 + year old boat what they designed it for.
In fact many of them had no engineered design, they built what looked right to owner of the company, was it right...who knows.

The only thing you can do is look at what was there (the glass portion).....see if it failed....if it didn't then you at least know that was good enough. (I don't count rotten wood as failing).

If you want to convert to epoxy, eliminate the mat in the original layup and you may be close, but on some boats all there was over the stringer was a light layer of mat, which meant the wood provided a great deal of the strength.

 

[erikgreen]

Re: The great debate -- epoxy vs poly layups

What can be said about poly vs. epoxy has already mostly been said, better than I could say it.

If you match the original thickness of glass on the stringers with your layup, you would be pretty safe. Strength wise you could probably cut thickness by about 1/3 using epoxy and directional glass(assuming the original glass on your stringers wasn't just waterproofing), but that won't save you very much money... just go ahead and match the old thickness and be confident of the extra strength.

If you want to use less glass and resin to minimize weight and maximize strength, you can use epoxy with an exotic fabric like kevlar or carbon fiber.. this will cut the thickness of the layup by more than half, and use correspondingly less resin. But the additional cost of the exotic fabric will more than outweigh the savings of using less epoxy.

What most people asking about this "debate" seem to want is someone to tell them that poly is "just as good" as epoxy for boat repair work. Other folks who have used poly for years seem to want some validation of the correctness of their work history.

"Just as good" is sloppy phrasing. If you ask a more precise question like "Which is stronger" or "Which costs less" then you'll get a better answer.

In general, polyester will work for most home boat repairs. It's cheaper than epoxy, and usually has less working time. It also isn't as good for making laminates and composite panels, but it's easier to work with for molding new parts, and it's not UV sensitive for the most part. It can be gelcoated. It's more porous than epoxy and needs gelcoat or paint coating for waterproofing.

Epoxy is more expensive than poly, is more flexible, is a much better glue (so it's better for making composite panels and laminating) and matches the yield strength of exotic fibers better than poly - that's only important if you need to build a boat that's going to get used in a way that actually requires that level of strength. I haven't seen one yet on this forum.

Personally I prefer poly for making new parts and I use epoxy for attaching new parts to old, for laminating composite panels (defined as a sandwich panel with glass on both sides and a core) and for laminating composite lumber (like stringers and transoms) together. I could use poly for everything or epoxy for everything.

I also recommend that people new to boat repair use epoxy at first for the advantages of long working time and lots of bond strength even if the surface is prepared less than well. The extra strength and adhesion are a nice safety factor.

Along with vinylester (another form of poly), poly and epoxy give us a toolbox of resins for different uses.

Which one you use for your boat work is up to you.

Erik