Dragging up the old epoxy vs poly argument

[77mutiny]

I know from reading old posts that you guys have gone around and around about epoxy vs poly. And as a new boat builder i had been under the impression that epoxy was the way to go, so i have been using it for my entire project so far, stringers and transom.
I've got my deck in and i have already put some strips of chopped strand across the joints where my deck boards meet, and along the sides where it meets the hull.
So now i'm getting ready to lay some combo chopped/biax on the deck and i was wondering if I can get away with using polyester for it? this will probably be the largest resin job of the entire boat, as i know how fast matt sucks up resin from doing the stringers.
From what i've read poly will not stick to epoxy, but i'm wondering if i can get away with it since i'll be covering short 4inch strips of epoxied matt with 6ft of poly soaked matt. i know i'm getting near the end of this project and am looking to spend as little more as needed.
i'm getting a little tired of paying 70$ a gallon for epoxy when i could get 5gallons of poly for 99$.
thanks in advance

 

[Mark42]

Re: Dragging up the old epoxy vs poly argument

Poly only makes a mechanical bond to epoxy. So where the poly will lay over epoxy, the trick is to get the epoxy roughed up so the poly has something to grip.

Seeing as the epoxy is 4" strips being coverered by 6 ft of biax, it probably won't be much of an issue.

Epoxy adds up fast, huh?

 

[77mutiny]

Re: Dragging up the old epoxy vs poly argument

thanks, that what i was thinking, hit the strips with a flapper disk and rough them up enough to get a decent bond.

 

[jonesg]

Re: Dragging up the old epoxy vs poly argument

Despite what mfers state I find anything works with anything, not quite as well and I wouldn't go mixing resins in a transom.
The blush should be removed from epoxy , the poly needs a tooth to grab.
If doing epoxy over poly, remove the wax on the polyester.

 

[erikgreen]

Re: Dragging up the old epoxy vs poly argument

Keep in mind that the mechanical bond from poly will be very weak. If you do the job perfectly you can expect a certain minimal strength, but if you've used epoxy so far, why switch now?

Personally I pay about $50 a gallon for epoxy with hardener, a bit less than that in bulk. For building my new boat I'm going to use about 14-15 gallons of the stuff. It's expensive compared to poly, unless you're looking at the really good poly resin. But if I'm spending the time to build the thing and the money on the other materials, the difference in cost is not that big a deal.

If you are gonna do this, I recommend you use epoxy to coat 3-4 test pieces, then glass over them with poly after trying out some prep techniques. You'll need to use mat with the poly between layers of fabric.

Test the pieces by leaving a corner or fold sticking up, and pull on it with a pliers when fully cured. That'll let you know if you did the prep right, and how strong your bonds are.

Erik

 

[leerocks]

Re: Dragging up the old epoxy vs poly argument

Keep in mind that the mechanical bond from poly will be very weak. If you do the job perfectly you can expect a certain minimal strength, but if you've used epoxy so far, why switch now?

Personally I pay about $50 a gallon for epoxy with hardener, a bit less than that in bulk. For building my new boat I'm going to use about 14-15 gallons of the stuff. It's expensive compared to poly, unless you're looking at the really good poly resin. But if I'm spending the time to build the thing and the money on the other materials, the difference in cost is not that big a deal.

If you are gonna do this, I recommend you use epoxy to coat 3-4 test pieces, then glass over them with poly after trying out some prep techniques. You'll need to use mat with the poly between layers of fabric.

Test the pieces by leaving a corner or fold sticking up, and pull on it with a pliers when fully cured. That'll let you know if you did the prep right, and how strong your bonds are.

Erik

Where do you get your epoxy?

 

[erikgreen]

Re: Dragging up the old epoxy vs poly argument

USComposites.com.

For the "thin/638" epoxy the cost is $270 for a kit with 5 gals of resin and 2.5 of hardener (2:1 for the "slow" hardener). So that's $36 a gallon plus shipping.

As far as quality, I've never had a problem with the stuff... no blushing, nothing. The same place also sells fabric, stir sticks, poly resin, vinylester, fillers, etc, although I go there for the epoxy.

They always pack really well when shipping too. My only complaint about them is that their ordering web page is a little primitive.

Erik

 

[Mark42]

Re: Dragging up the old epoxy vs poly argument

Think twice about using non-blushing epoxies. They are considerably weaker than their blushing counterparts. I could not find the strength listed for the west system thin, but all other brands show about a 20 -33% reduction in strength between their regular blushing and non-blushing epoxy.

 

[erikgreen]

Re: Dragging up the old epoxy vs poly argument

Mark -

I don't think you can make a blanket statement like that about non-blushing epoxies. It's true that epoxy hardener with additives to prevent blushing can change the composition of the final resin, but it's also true that blushing can be more affected by cure times, CO2 and humidity than anything else.

The big change that can happen with a hardener containing cyclic amines (for blush avoidance) is that the final resin has a higher modulus of elasticity... that is, it will flex farther without cracking than a blushing, fast hardening epoxy. So it can be said that it's "weaker", but it's more accurate to say it's "different".

This same resin can be used with slow, medium, and fast hardener, and only the slow is advertised as non blushing. I've never had any blushing with any of the hardeners, though, possibly due to the other factors.

Actually, the US Composites resins are rather slow compared to eg. west... with the West fast hardener I had to work in less than a minute or two or else I'd get smoking epoxy in a cup. With the USC fast, I could work for 5 min before it started to stiffen.

Anyway, all the informal testing I've done says the USC epoxy is as strong as the West... and it's hard to beat the price. Try it if you haven't, it's good stuff. If you must avoid non-blushing, buy the medium or fast hardener, it's still plenty slow.

Erik

 

[Mark42]

Re: Dragging up the old epoxy vs poly argument

I haven't used those brands of epoxy, and right now its not a good idea to change brands in the middle of a project. But next epoxy project I might try them.

But two different suppliers told me not to use their non-blushing epoxy due to strength issues for my hard top project. I noticed that the one you mentioned above, the "thin" epoxy I think, did not list strength specs. But the brand epoxy I have used does and there is a big difference when using blush vs non blush.