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PostPosted: Sat May 02, 2015 2:17 pm 
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epoxy vs polyester vs vinylester, what to use when and where?

I've heard a lot about each, but I've also heard some conflicting recommendations. So which is best to use in which case? Hull damage/repair, soft spot repair, bottom repair and EPO repair, which is best?

I understand that epoxy is the hardest and more abrasive resistant than the other two, but also more flexible. However, I've heard it may create a hard spot in the hull when used on a soft spot, potentially creating issues with the flexing of the hull. I've also heard that polyester/gel coat does not adhere well to epoxy or may crack if applied on top of epoxy, as it isn't as flexible - hence why it's not recommended for hull repairs.

Polyester, I understand, is what the hulls were originally made with. This is the recommended resin for hull repairs, as it will have the same strength/flexibility as the surrounding material. It will also allow proper adherence of gel coat. This is the least expensive of the three, but has a tendency to absorb water when exposed. It does not bond well to epoxy. (Note that gel coat is made with polyester resin, but is more resistant to water absorbtion)

Vinylester is the resin originally used in EPO's I believe, and is more UV resistant than the other two. For that reason, hobie hotline recommended vinylester for EPO repairs. It is also the strongest and most expensive. However, the youtube how-to video by Jeremy at Surf City Racing recommends the use of epoxy throughout.

So, hull repairs get polyester, soft spots get epoxy, bottoms get epoxy for abrasion resistance or polyester for compatibility with gel coat, and rudders get vinylester?

EDIT: I have made minor corrections and clarifications to the information above, for the benefit of anyone referencing this thread in the future.

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Last edited by SabresfortheCup on Mon May 04, 2015 8:52 am, edited 1 time in total.

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PostPosted: Sun May 03, 2015 8:07 am 
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Hard to reach spots, or areas of wear (corners of dagger wells) = epoxy.
Epoxy eventually has a problem with UV, unless it's painted over.
So for those 'hidden' spots, which are difficult to reach, it is perfect.
Epoxy is stronger, but can be more brittle, which means you have to be careful as to where you apply the epoxy if that is a 'moving' or load bearing part of the boat, especially if it is in an area where the two different composites might 'shear'.
If your repair is that big, you might want to get a professional's opinion....

Main parts of hulls etc = polyester (that's what the H18 is made of)

EPO's = I know very little about...

Just downloaded the Club's SI's for the year, can't wait to get on the water.

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PostPosted: Sun May 03, 2015 9:17 am 
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Thanks John, that's what I had suspected for hull repairs. My current projects are a soft spot on the inboard side of the port hull, alongside the daggerboard trunk (for which I'll use epoxy), EPO rudder resurfacing, and a bottom job I've been putting off. Maybe I'll get to it this year! :?

I've heard that you want to use polyester & glass on the bottoms so that it doesn't change the bending/strength characteristics of the hull, and especially if you're going to put on some gel coat (I'd like to, but we'll see), but I've also been told epoxy is better there as it stands up to the abrasion better.

My most immediate project though is the EPO's. I bought what looked like a very nice pair, except they were covered in black appliance paint (enamel?). I wasn't crazy about it, but I figured better than the standard plastic. The paint started peeling almost immediately, due to poor surface prep, so I'm stripping them down and repairing them properly. Wow are they bad. Glass fully exposed on one entire face, both rudders. However, I just noticed that the hotline article recommends vinylester, while Jeremy at Surf City just uses epoxy in his YouTube how to, so I'm curious what others have used. Either way, being that the glass is so exposed, I figure I'll apply three coats.


Last edited by SabresfortheCup on Tue May 05, 2015 12:41 pm, edited 1 time in total.

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PostPosted: Mon May 04, 2015 4:50 am 
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You can bond epoxy to polyester. You can not reliably bond polyester to epoxy. So if you plan on using polyester resin in the future (i.e. to gelcoat the repair), then you need to use polyester resin. Since the boat is made from polyester resin, it is safest to use polyester. That being said, epoxy has superior mechanical properties, so it is "nice" to be able to use epoxy in certain applications.

My general rule of thumb, if it is a repair that is internal to the hull, then I use epoxy. If it is a repair that is external to the hull, then polyester. So for a bottom job, I would use polyester. To rebond a hull seam, repair delamination, or do an internal structural repair, I would use epoxy.

For an EPO rudder repair, I would use epoxy. I believe the EPO rudders are made using epoxy resin (hence E P O).

Advantages of polyester - low cost, no compatibility issues with existing hull material, easy to use (precise mixing not required).
Disadvantages of polyester - inferior mechanical properties compared to epoxy, nasty fumes, can not be used over epoxy.

Advantages of epoxy - superior bond strength, superior mechanical properties, easier to tailor cure time by using various hardeners, works over polyester or epoxy.
Disadvantages of epoxy - accurate mixing is required, higher cost, can not be reliably gelcoated, UV sensitive.

sm


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PostPosted: Mon May 04, 2015 7:45 am 
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Thanks srm, I was leaning towards using epoxy, I just wanted to see if anyone strongly recommended vinylester instead. It seemed to me that most people used epoxy.

I had thought they were originally epoxy as well, but in his Hotline article "Fine Tuning Rudders" (http://www.w1dm.com/projects/hotline/11 ... OTLINE.pdf), Matt Bounds suggests that the rudders were originally made from vinylester, which is apparently "produced by the esterification of an epoxy resin with an unsaturated monocarboxylic acid" (thanks wikipedia :wink: ). So I suppose that still makes sense? Supposedly the Vinylester is considerably stronger and more UV resistant than polyester and epoxy, although softer, and is much less viscous as a resin. It's also supposed to be far more resistant to water absorption and corrosion, though I expect epoxy isn't very absorptive on it's own either. It's much more expensive, less common, and as a result, harder to find.

My understanding is that the graphite helps with the UV resistance, but not that much (perhaps just by preventing the UV from penetrating further into the epoxy?). I'm considering a can of white spray paint for a topcoat. The black paint got very hot in the sun, which I think contributed to the peeling issue. First they bubbled, then with use, they peeled. But definitely not enough surface prep beforehand.


Last edited by SabresfortheCup on Tue May 05, 2015 12:41 pm, edited 1 time in total.

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PostPosted: Mon May 04, 2015 9:51 am 
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When I've reworked original EPOs, my process has been:

Rough sand surface to remove all loose fibers and give the epoxy something to bond to.
Apply a layer of epoxy to each side (one side at a time).
Sand the epoxy.
Apply a second coat of epoxy if necessary and sand.
Spray paint with flat black krylon, two coats.
Wet sand up to 1500.
If you blow through the paint, repeat the process.

Note that a sanding sponge works really well for achieving a smooth contour.

sm


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PostPosted: Mon May 04, 2015 4:24 pm 
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srm wrote:

For an EPO rudder repair, I would use epoxy. I believe the EPO rudders are made using epoxy resin (hence E P O).



sm



Expanded PolyOlefin

It refers to the type of foam used in construction of the rudder. :wink:

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PostPosted: Tue May 05, 2015 12:41 pm 
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What's so special about Expanded Polyolefin? I wouldn't think the specific material of the foam core would be all that notable in this type of application.


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PostPosted: Wed May 06, 2015 7:51 am 
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SabresfortheCup wrote:
What's so special about Expanded Polyolefin? I wouldn't think the specific material of the foam core would be all that notable in this type of application.

The original EPO rudders were built using technology from O'Brien Skis. Both Hobie Cat and O'Brien were owned by Coleman Co. at the time (1984-1988). The difference in core material was in how the core was incorporated into the blade.

In conventional construction, the foam "cookie" is pre-cut and laid into the mold with a generous slather of thickened resin. The mold halves are clamped together while the layup cures. The problem with this method is added weight and the high probability of entrapped air (voids).

The EPOs were made with an expanded foam - a precise amount of catalyzed liquid foam was placed into the mold before the two halves were clamped together. The foam expanded to precisely fill the cavity between the halves. Getting it to come out right was tricky - and the knowledge/technology to do this was lost by Hobie Cat when Coleman sold the company in 1988.


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PostPosted: Tue May 26, 2015 8:16 am 
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Very interesting stuff, Matt! I can't imagine how you'd go about the quality control in that process! Fascinating stuff, though!

I'm impressed with how well the epoxy & graphite repair is going, really seems effective. I'm sure these rudders haven't been in this good of shape in decades! Pictures below.

Image
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Stripping off the old paint. The other side was peeling horribly, but this side actually seemed well adhered. Once I got the blade under it, however, it still peeled pretty easily.

Image
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Completely stripped down and wiped down with acetone. I was shocked at how much exposed glass was hiding underneath the paint!

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Definitely original EPO's!

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First Coat of Epoxy applied

Image
Image
Second coat applied and sanded down. One more coat should make them like new!


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PostPosted: Tue May 26, 2015 8:46 am 
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Now, I've read in a few other threads that people recommend putting a final coat of spray paint on the rudders to protect them from the UV, as the epoxy alone doesn't stand up well and the graphite has limited effectiveness at it. I imagine you'd have to sand to ~120 grit, spray paint, then sand that down to your desired grit. I'm curious how the spray paint holds up, though? The whole reason I'm resurfacing my rudders is that the previous owner did a poor job painting them, and the paint was peeling off. It took me quite a long time to strip all of the old paint off the rudders so that I could re-coat them in epoxy, and I don't really want to run into the same situation. Even if it adheres well, I imagine that whenever they need to be re-coated, all the spraypaint will need to be stripped off/sanded down? So I'm curious, what type of paint (epoxy, enamel, etc?) have others used in the past, how did it hold up, and what happened when it was time to re-coat them?

My second question relates to the bottoms of my hulls. One hull must've been damaged in the past, because there is a large spot of Marine Tex (epoxy) repair on the bottom of the hull, just forward of the daggerboard trunk. I see no evidence of the repair on the inside of the hull, but looking on the outside, it looks pretty thick. I'd guess about 1/4" (there are pock marks/bubbles ~1/16" - 1/8" deep). I know I can't put glass & polyester over it, so I guess I'm trying to determine how worthwhile it would be to try and chip it all out and replace it with glass, or just leave my bottoms as-is. I'm worried about turning it into a bigger job than it needs to be or compromising the strength of my hull. The Marine Tex seems to be holding up, and I don't want to create a problem where there isn't one. Anyone dealt with something like this before?

Image
Image


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PostPosted: Wed May 27, 2015 5:30 pm 
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Maybe there is a third material that would bond adequately to both Marine-Tex and gel coat. Formula 27, maybe? Then you could apply a thin coat of it over the Marine-tex and then shoot the whole thing with gel coat. If not, I would say shoot the whole thing with gel coat and hope for the best over the Marine- tex spot. It doesn't matter how the bottoms look, only how smooth they are.

Jim


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PostPosted: Thu May 28, 2015 6:17 pm 
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I doubt I'll be able to just cover it over, I just don't think anything will be able to adhere to the epoxy.

I guess the question is whether or not the original repair is sufficient as a lasting/permanent repair, if the bottoms are bad enough to absolutely require new glass, and how much better it would be if I chipped & sanded it out and made the proper repair. It seems that a fiberglass repair in and of itself would be simple enough to do, but I hadn't previously considered the presence of the MarineTex complicating the process. As much as I don't want to, I'm considering chipping it out and putting down new glass. I'm just nervous about potentially doing more damage than good. I really don't want to wind up with a hole in the bottom or a weak spot in the boat.

And because I'm a (cheap) perfectionist, I'm thinking if I go through with it, afterwards I'll roll on some gel coat, sand it down, then use some rubbing & polishing compound. (The whole boat could use it).


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PostPosted: Fri May 29, 2015 2:37 am 
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You have an access port right there. What does it look like from inside the hull? You could very easily add a couple plys of glass inside the hull if you're concerned about it.

sm


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PostPosted: Fri May 29, 2015 7:02 am 
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No real evidence of it from the inside, so it doesn't go all the way through. It doesn't stand proud of the hull, so chipping it out would result in a "crater" in the hull that I'd have to fill back in with glass & resin, and then fair.

Image
Image
Image

The port hull is probably a little more in need of new glass, with 58" x up to 3" exposed forward and 32" x up to 2" aft, vs. the 62" x up to 2.5" exposed fwd and 2" x 14" aft on stbd hull (The Marine Tex is on the Starboard hull)

I'm figuring a strip of 2" tape on each hull is necessary, followed by maybe a 1.5" wide strip of mat. I'd think that should build back a little shape and add a little strength. The port hull may need a wider strip of mat beneath the 2" tape. I'm hoping a quart of polyester resin and a pint of finish gel coat will be enough to cover it, all applied with brushes and rollers (I don't have a spray gun, but I figure I can roll it on relatively smooth and then sand it as needed).


Last edited by SabresfortheCup on Fri May 29, 2015 7:33 am, edited 1 time in total.

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