Poly over Epoxy what West System says....

[Mark42]

This is taken from a West System docuemnt:

In repairs above the waterline, gelcoat applied
over properly prepared WEST SYSTEM epoxy
has a great track record. Many boats have
been repaired this way and are performing
well in a variety of climates. Boats that are dry
sailed or trailered have also had good success
with gelcoats on underwater epoxy repairs.
Concern about adhesion occurs in situations
where boats are in the water constantly and
need to have a high level of finish on their
bottoms that requires gelcoat. Such a situation
might occur with a boat kept in a pristine lake
where antifouling paint is often not used.
Since the polyester gelcoat is not very good at
preventing moisture from permeating through
to the laminate, there has been a concern that,
over time, the gelcoat will detach from the
more moisture resistant epoxy layer.
.....

Conclusion:
The overall effect of moisture on the gelcoat?s
bond to both the polyester and epoxy panels
was negligible. The graphs show very little
change in the adhesive strength, and much of
the variance is within the tolerance of the test
method. When the studs were pulled, the failures
occurred in the fairing compound, cohesive
failure of the gelcoat, or in the bond
between the gelcoat and laminate. Since these
three modes of failure occurred under similar
loads, it indicates that the strength of the bond
is close to the cohesive strength of both the
gelcoat and the fairing compound

Read the entire article here: http://www.westsystem.com/ewmag/22/pdf/Ew22_Polyester.pdf

 

[Stoutcat]

Re: Poly over Epoxy what West System says....

Hi Mark42,

The full article is a great read, but I fear that the West folks may have failed to use the appropriate tests... They used PATTI studs they'd applied to static epoxy panels... The problem might then become, what of non-static (flexing) epoxy?

From the article:
Applying gelcoat to a cured laminate relies on
a mechanical bond. Because of the difference
in curing chemistry, it is not possible to
achieve a chemical bond between epoxy and
polyester gelcoat. We developed some tests, to
determine whether or not the mechanical
bonds achieved between gelcoat and properly
prepared, cured epoxy were strong enough to
achieve a durable repair.

My concern is that a trailer fg boat will have an epoxy repair which will then be gelcoated... If the repair was in an area that is commonly put under the flexing of trailer rollers and beds, the mechanical bond between the gelcoat and the epoxy repair might fail. A good analogy might be that of cured epoxy in a plastic tub... The mechanical adhesion is great until you start to deflect the sides of the tub... Once the mechanical adhesion is gone, there is no chemical adhesion to "hold the fort" and the epoxy and plastic easily part ways.

I guess I'd just like to know more about just how good this mechanical connection is under various stresses...

Interested to see other comments/experiences on this.

Alan

 

[Mark42]

Re: Poly over Epoxy what West System says....

I found that article while searching for epoxy fairing compound to use on my Hard Top project. Epoxy fairing is so expensive ($150/gallon) compared to lightweight polyester based body filler ($20/gallon) that I am hard pressed to pay the epoxy price.

What got my attention was that the poly gel coat has good adhesion to both cured epoxy and cured polyester fiberglass. It does not make a chemical bond with cured polyester fiberglass, same as with the cured epoxy. This got me thinking that polyester body filler works the same way. It does not have a chemical bond to the auto it is being applied to, or even the polyester fiberglass work on a boat that has fully cured.

I'm going to do a test with a body filler on epoxy to see how well it adheres. It if sticks well, I may use it as the fairing on my hard top project where the filler will not be below the water line.

I'll keep you posted on the results.

 

[Bondo]

Re: Poly over Epoxy what West System says....

Epoxy fairing is so expensive ($150/gallon) compared to lightweight polyester based body filler ($20/gallon) that I am hard pressed to pay the epoxy price.

Ayuh,....

That's why those of Us who use Epoxy just build our own Fairing mixes....

Epoxy resin with micro-balloons,+ abit of cabosil is a Great mix that's still quite Sandable...

 

[Mark42]

Re: Poly over Epoxy what West System says....

Ayuh,....

That's why those of Us who use Epoxy just build our own Fairing mixes....

Epoxy resin with micro-balloons,+ abit of cabosil is a Great mix that's still quite Sandable...

The problem there is you are still spending $100 plus for a gallon of epoxy to mix with the fillers. I did find one supplier that sells the epoxy fairing at the same price as the epoxy.

 

[Bondo]

Re: Poly over Epoxy what West System says....

Ayuh,....

Have you tried Larry at raka.com

That's where I've been spending my epoxy money the last couple of years...

Btw,....
That Gallon of epoxy really goes Along ways when you start mixing fillers into it.....

 

[Mark42]

Re: Poly over Epoxy what West System says....

Bondo, they do have good prices. I couldn't find their shipping fees, will give them a call Monday.

I am surprised at what I'm finding about poly over epoxy once I started doing searches on the topic. I just always went with the prevailing wisdom that poly does not stick to epoxy. Apparently that is conditionally true. Here is a little excerpt from a autobody man with 30 years experience about using bondo over epoxy repairs:

The resin used in Rage and other body fillers is fiberglass (polyester) resin. Today's resin technology makes the adhesion of these new resins as good as or better than most epoxy adhesives. They are designed to really stick. Also, they are somewhat flexible when used properly.

Most, or probably all, filler manufacturers design and recommend them to be used over clean and prepped (sanded with coarse 36-grit), rust-free (sandblasted), dry (free from moisture), bare metal. They will stick to epoxy primers with mechanical adhesion. Body fillers do not chemically bond with epoxies. If you put filler over epoxy primer, it has to be fully cured, with no solvents such as thinner or reducer left in the primer, and still should be sanded with a coarse grit and cleaned. In other words, using epoxy primer under filler is a wasted step that may or may not cause adhesion problems later on.

One of the most critical points is for everything to be clean and moisture-free.

Source: http://www.roadsters.com/filler/

So far, everything I have read indicates that poly will work over epoxy as long as the surface is properly prepaired.

Here is a test by a auto body professional testing the adhesion of bondo to epoxy. His tests show that the bondo sticks to epoxy better than epoxy sticks to metal. Not sure how valid his tests are, but its a short read with lots of photos: http://www.autobodystore.com/filler_&_epoxy.shtml

 

[ondarvr]

Re: Poly over Epoxy what West System says....

This has come up before, when it did, I contacted West Systems myself to discuss it, I also reviewed it with our chemists to make sure nothing had changed over the years.

Our chemists to a person, said using polyester over epoxy is a poor idea, and I can say that I've personally tested the bond myself on very well prepped surfaces and found the bond to be less than satisfactory also.

When I discussed it with the West Systems technical rep, I told him who I was and the company I work for. We discussed the claim they make and while he did say with properly prepped surfaces the bond was fair, he also indicated that in the real world it may not be the best method and probably wouldn't hold up as well as staying with one type of resin.

Yes you can get it to stick, but that doesn't mean it will stay in place when stressed, plus there are many different formulations of epoxies and polyesters, so some "may" do a better job of bonding than others.

Currently no polyester gel coat or resin manufacture recommends it, really only W/S does and I know of no company that will use these products in this manner.

In my testing the failure was always directly in the bond line between the two products never in the epoxy substrate or in the polyester coating. In a typical bond test using only polyester products you will find the failure point will be in a combination of substrate, bond line and coating (gel coat or polyester resin and/or laminate), the better the surface prep the more this failure point will vary between them. On a poorly prepped surface, or when non compatible products are used, the point of failure will more likely be only at the bond line.

 

[BillP]

Re: Poly over Epoxy what West System says....

WEST may be one of the gods of epoxy but some of their testing is just plain narrow (and done to sell products?). Any decent statistical analyist would shoot their empiracle testing down as unreliable and inaccurate. WEST also says PT wood doesn't work on boats but that is proven to work industry wide for several decades now. By the way, WEST's untreated ply was the worst wood I've ever bought. It delaminated while stored in the rafters a dry shop. Not even cheap interior ply from homedepo does that.

The real problem with poly over epoxy is in a timeline. Sure, poly adheres for "some" time but the odds are against it lasting long term. Who knows how long? Same as WEST tests, bodyshops don't see the long term and don't really know what was previously used on autos that come in for work. They are probably basing adhesion tests on a few yrs...maybe.

What I always consider on this topic are the chemists & mfgs who formulate poly say not to use it over epoxy. For whatever reason they have I don't second guess them. If you can find a poly mfg that says it's ok I'd use his product...good luck finding one though because the industry say not too. Otherwise its a crap shoot.

bp

 

[ondarvr]

Re: Poly over Epoxy what West System says....

It's not uncommon to see the body filler fail within itself and not at the bond line. The fillers are normally the weak link and there is often a slightly resin rich layer at the bond line making this area slightly stronger than the rest of the filler. Body fillers tend to be somewhat flexible also, so that when the metal does flex it won't just break. The type of resin used, plus the easy sanding fillers added to it are typically less water resistant also, so in wet environments they don't hold up as well. For a price all of these ingredients can be upgraded for better performance though.

 

[redfury]

Re: Poly over Epoxy what West System says....

Well, if this argument falls within the guidelines of the argument made in my industry about water based and oil based polyurethanes, I have some experience/knowledge on it personally.

You can skip to the last segment if you don't feel like reading my diatribe

The prevailing wisdom was that water based poly would fail over oil based/modified finishes/stains or not adhere ( separate/fish eye ). This is true to the degree that if the solvents haven't been properly released from the oil based finish before applying the water based finish, you would end up with an inferior end product.

However, we've been putting water over oil and oil over water for quite some time. What I've found more than anything is that the prep work done between coats is the critical issue. The manufacturers say you can coat each type of finish directly on top of the previous coat if it hasn't dried thoroughly...meaning that the solvents/chemical reactions haven't completed their cycle. Well, who knows exactly when that happens IRL? I've seen more problems with water based finishes peeling off of water based finish than I have seen with oil based finishes.

Water based is technically a harder finish and can be formulated to be almost as strong as the oxide finishes they put on prefinished wood using a UV cure technique...but it has less flexibility.

A stronger resin may not necessarily be the right resin for every application.

A question though about the gel coat question....what do boat manufacturers use in place of a gel coat if they build an epoxy hull?

 

[i386]

Re: Poly over Epoxy what West System says....

A question though about the gel coat question....what do boat manufacturers use in place of a gel coat if they build an epoxy hull?

This is purely speculation on my part but I would guess they use a pigmented epoxy resin instead of poly.

 

[ondarvr]

Re: Poly over Epoxy what West System says....

Epoxy parts are typically painted, although we do make bond coat products to go between epoxy and gel coat. These products aren't that popular though.

Epoxy has poor weathering properties, so even when tinted with pigment it doesn't do all that well in the sun.

Oil and water based paints have little to do with this, the chemistry is far too different.

 

[redfury]

Re: Poly over Epoxy what West System says....

I was just speculating about the differences between the two...( epoxy/poly resin vs Oil based and water based finishes ). I know that what I work with are "coatings" and the discussion is about "resins"....

 

[i386]

Re: Poly over Epoxy what West System says....

Epoxy has poor weathering properties, so even when tinted with pigment it doesn't do all that well in the sun.

That makes sense. I didn't think about that.:redface:

 

[Mark42]

Re: Poly over Epoxy what West System says....

Well, I'm not fully convinced that using poly filler will work all that well.

One reason I am looking into the poly filler (besides the price) is that when I tried mixing the micro balloons with epoxy I ended up with a paste that sagged as it kicked off.

I mixed the resin first, then poured in the micro balloons until thick. Maybe I should try mixing on a flat surface rather than in a cup.

I'll try mixing up 4oz of resin and try again.

Any suggestions on how to mix micro balloons until the resin is more like bondo?

 

[i386]

Re: Poly over Epoxy what West System says....

Well, I'm not fully convinced that using poly filler will work all that well.

One reason I am looking into the poly filler (besides the price) is that when I tried mixing the micro balloons with epoxy I ended up with a paste that sagged as it kicked off.

I mixed the resin first, then poured in the micro balloons until thick. Maybe I should try mixing on a flat surface rather than in a cup.

I'll try mixing up 4oz of resin and try again.

Any suggestions on how to mix micro balloons until the resin is more like bondo?

I use 3M glass spheres instead of microballoonns for whatever that's worth. After my resin is mixed well, I start working in the glass spheres. I keep adding it until it's about the consistency of sour cream. At this point it feels light and whipped up but it will still run off the stirring stick. Then I will mix in Cabosil until it thickens up enough to not run off the stick.

The glass spheres by themselves will not sufficiently thicken the mix. If you keep adding it past a certain point it will get a grainy/sandy texture to it.

Once it cures it will knock down pretty easily with 80 grit and finishes nicely 220.




By the way. I use US Composites epoxy resin 635 thin with slow hardener. I use the pumps 3:1. Never had any problem with the pumps either. Once they're primed, I never get air bubbles even after sitting for weeks. Anyway, my typical batch is 3 pumps resin, 1 pump hardener. Then it's about 6-7 heaping plastic spoons of glass bubbles. That's followed by 2-3 heaping plastic spoons of Cabosil. I always add fillers one spoon at a time and mix them completely in before adding the next spoonful.

 

[Bondo]

Re: Poly over Epoxy what West System says....

Any suggestions on how to mix micro balloons until the resin is more like bondo?

Ayuh,.... i386 has it Nailed.....

The microballons are for sandability,...
The Cabosil is what stops the running,+ sagging.....

Just glassballs,+ it runs....
Just Cabosil,+ you need a Grinder to sand it.....

The glass spheres by themselves will not sufficiently thicken the mix. If you keep adding it past a certain point it will get a grainy/sandy texture to it.

If you can find the Maximum amount of glass you feel is comfortable,...
That's where you can get the mileage out of your Resin....

 

[Mark42]

Re: Poly over Epoxy what West System says....

Ahhh. I see. Gotta get me some cabosil!!!!

So I guess it works better when all the ingredients are in the mix?


Thanks!

 

[Mark42]

Re: Poly over Epoxy what West System says....

Anyone use a product called EZ-Thick to thicken their epoxy?

EZ Thick:

An alternative to the every popular Fumed Silica, EZ thick is a cellulose based powder that quickly and easily blends into the epoxy (unlike fumed silica). A 1 gallon unit will thicken about 3 quarts of our No Blush epoxy into something approaching mashed potatoes (in texture and color).


1 quart - $4.00

VS Fumed Silica (aka Cabosil) that looks like this:

Th