No polyester on top of epoxy?

Just wondering. Why can you apply epoxy onto polyester, but you can't do it the other way around?

At the end of the day, it's a layer of epoxy against a layer of polyester. Right?

 

It isn't fair, but you'll have to live with it. The world frequently fails to conform to our expectations.

 

What about outer space? There's no "up and down," and no "top or bottom."

Does this rule still apply in outer space?

(seriously, what's the deal with that?)

 

polyester resin doesn't 'stick' very well to anything but itself. That's why fiberglass boats are 100% fiberglass and not a wood/glass mix.

paul oman
progressive epoxy polymers inc. epoxyproducts.com

 

Expanding on what Paul said, when inspecting older boats built in the 70s and 80s it's common to find polyester FRP over wood bulkheads and small items including hatches, seat components, and of course transoms and stringers. The fiberglass is thick enough over transom and stringers to hold its own shape. Where a layer of polyester fiberglass has been applied over bulkheads or small items such as rear seats though, especially large flat panel areas, it's common to find some areas where the fiberglass skin is no longer attached to the bulkhead and there are areas that have an air void between the skin and wood with time.

 

liquid epoxy will make a mechanical bond to cured Polyester resin but the poly won't make a viable mechanical bond to the epoxy. It won't even make a very strong bond to itself when applied wet on cured.

 

The two types of resin are very different chemically, and epoxy is a very good glue, it tends to bond to most substrates quit well. Polyester isn't a very good glue and isn't designed to be used in that way. While a chemist could into great detail on why, the final answer is still the same, polyester just doesn't like to bond to epoxy.

 

Polyester does stick to epoxy, though not as well as itself, it's still noty a bad bond, unless making comparisons between the two. As a rule, you'll want epoxy over polyester (as mentioned) and try to avoid polyester over epoxy, though it is often done anyway. In high stress and high vibration areas, the polyester over epoxy bond will fail quicker, because of elongation modulus differences.

 

It is said that in the divine realm, there is no here and no there, no now and no then, no mine or thine. Makes polyester seem simple.

 

Kind of depends how "not a bad bond" is defined.

I've never been able to get a bond I trusted, and I've tested it many times. It also depends on the exact epoxy and exact polyester used, there are many versions of both and some may be more compatible than others.

So in light of the possible variables in bond strength, we as a company don't recommend it, and I personally don't recommend it either.

There are bond enhancement products that can work very well, but they tend to be pricy and sometimes hard to find, so it's normally easier and cheaper to just stick with epoxy (pun intended).

 

Maybe improved by "keying" the surface ? I can't imagine polyester gripping to a slick epoxy surface with any reserve at all.

 

My testing was done on well cured epoxy and using 36 and 80 grit grinding discs

 

What happens when you apply polyester to a part-cured epoxy ?

 

Yeah, that's the butt kicker and we're in the same boat, as anyone that's read my thoughts on the subject previously. I don't recommend it for anything other than small cosmetic applications, certainly no load, stress or structural considerations.

Over green epoxy doesn't help, in fact it will usually cause a problem. It only works over well toothed epoxy and though some "tie coat" products are available, it's still not something I'd trust in loaded, structural applications, nor over large areas (though I've seen this done). Yep, these tie coats do tend to be costly, but if you're trying to apply gelcoat over large areas, possibly justifiable.

 

vinyl ester to the rescue

I have just recently tried vinyl ester resin over green epoxy trying for a bond. No go, the vinyl ester never cured. As for scratching the epoxy with 80 or even 36 grit no go here either you still don't get any kind of what we call a burn in bond. We now use vinyl ester resin only for building hulls, decks and parts and repairs to poly ester boats. So much better than the old standby poly. It also sticks much better to wood providing it is applied correctly. I have even messed around with priming the wood with a thinner infusion resin so it penetrates the wood better. By the way, what is your application? What are you covering?................peace out