When is epoxy a bad choice?

I am still just getting my sea-legs when it comes to epoxies and resins. Am I correct in my understanding that epoxy is much more brittle in general than your basic "boatyard" polyester resin? This seems correct from my experience with both. If so then,for you guys with experience...

What are some situations where the brittleness of epoxy makes it a bad choice?
Is this really only a worry under high shear loads where the two bonded areas could bend in different directions like corners? Will applying the epoxy in thin layers help this problem? I don't have a specific question regarding a project but am just wondering. Thanks!

 

Epoxy is less brittle than poly.

 

True - and its the cloth that provides the significant sheer load strength no matter what 'goo' is used.

 

Epoxy is a better resin system in every way you can measure the differences between the two. I can think of no example of wanting to use polyester instead of epoxy.

 

Hmmmm -I can Par, when Cost matters, Polyester is waaaay better. Mind you, I wouldnt go lower down the food chain than VinylEster for a hull.

 

I chopped the cabin off an Alacrity 19 cruiser today in order to make it into a daysailer with a cuddy. This is particularly easy because the cabin is stepped and the forward part remains on the boat as a cuddy.
Why chop off the cabin (and the cockpit too)?
In a word, polyester resin. Virtually all of the structural and non-structural tabbing had let go when the boat sat with water in the bilge and the sun cooking it.
To get under the cockpit, let alone crawl into a small cabin to grind and rebuild would have nixed the whole rebuild project due to extreme hours of labor, but changing to a non-self-bailing cockpit from the original one gave access to the whole insides.
If epoxy had been used originally, I wouldn't have been able to afford the boat. The hull is absolutely like new (a testimonial to the proper use of polyester--- in large monolithic structures). Even the polyester gelcoat has taken sanding and buffing to the extent that it's hard to believe the boat is at least 40 yrs old.
The prudent use of both polyester and epoxy will yield the best of both worlds.
The tabbing in this boat in particular was very complicated and involved many wood parts. The parts were not totally encased in poly resin, but were allowed to take on water from other non-coated surfaces. This is what caused the tabbing to let go, but also actually reduced rot in the majority of the wood because for the most part, the wood could dry out once the water level in the bilge receded. Cutting through the overhead stringers (no balsa or ply coring, thank God), the wood was dry and strong.
The tabbing ought to have been done with epoxy. Each wood part attached should have also been sealed (though had it been polyester resin in the tabbing, sealing would have spelled doom for most all of the wood against the hull.
Bottom line, epoxy is just the right resin for building a boat that can stay rot-free for more than a quarter century, and probably longer. The hull is ployester, but any wood attached to it should be encapsulaed and bonded with epoxy.

 

Okay..well ..cost is a factor..will use resin on cabinetry areas for sure but flexibility was where I was really unsure of the properties of these two goos...I have it straight now... thx folks...btw..Alan...I am right behind you with my sawzall and a 1983 Hunter 20...oh..and a midget holding a stiff drink.

 

Alan...some pics of your project would be incredible...and no I am not really that close to hacking into my Hunter but I am curious how yours looks before /after

 

I took a couple today, very gruesome! It took about an hour and a half. Chose the coaming material also, will pick up tomorrow, two book-matched pc 1/2" x 8 3/4" x 10' african mahogany. The seats and sole will be port orford cedar from a local yard.
I'll take some pictures as I go along and post them here.

 

The main drawbacks with epoxy are alergies and cost. I am alergic to both the epoxy and the cost but persist in using it for its superior bonding properties. I need to don a chemical suit in order to avoid any adverse effects.

I would also avoid using epoxy where there will be prolonged exposure to sunlight as it deteriorates under UV bombardment.

Epoxy with its superior bonding properties makes it the only option in my mind for composite construction. Epoxy bonds extremely well to polyester and vinylester but not necessarily the other way around. So don't use epoxy in a situation where you will need to bond polyester to it in the future. Some paints and finishes also take exception to bonding to epoxy so consider the project as a whole when using epoxy.

 

When is epoxy a bad choice?
When clear epoxy is eposed to UV.

Brittle??
I am current using a specifically designed epoxy with a 50% tensile elongation.
Don't know of any modified poly that will do that.

 

Yes, there are polyesters that can do that as well. Mostly used for roofs. Not for construction work.

One other factor that must be kept in mind: Temperature resistance. In general, polyesters have a higher Tg then epoxies. (but keep in mind that both polyesters and epoxies are available with a Tg of over 200 degrees C)

 

Back to this thread for awhile...if I go with epoxy for my new coach roof on my Hunter 20 what do I need to consider as far as UV which I understand epoxy or clear epoxy cannot withstand well...is there a pigment added that counters the UV or does the right kind of paint over the epoxy protect it from the UV which a coach roof would be highly subject to?...thanks in advance.

 

Good question, I look forward to a good answer.

One heads up: Five Minute Epoxy is markedly more brittle than it's slow cure counter-part.

Cheers, Tom

 

On the 5 minutes epoxy: Depends on the brand. What they sell here is very flexible.

Coachroof: Apply epoxy as you are familiar with, then coat with a 2part PU paint with UV filter, minimum 2 coats.