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PostPosted: Thu Mar 29, 2012 9:18 am 
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Five years ago I did a West system epoxy bottom job on my boat, the remains of which are easy to see (and remove) as the epoxy is badly yellowed against the white gel coat hulls. It's time to do it again but I was wondering if I should go with polyester resin instead of epoxy this time. I drag my boat on and off a sandy beach so I doubt either system would be any more durable. I was just wondering if there are any other advantages of one over another or should I just go with the cheapest?


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PostPosted: Thu Mar 29, 2012 9:30 am 
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Polyester will be cheaper and easier to deal with.

And it won't amber with UV exposure like epoxy does.


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PostPosted: Thu Mar 29, 2012 11:05 am 
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Sounds good. Polyester it is then. A friend told me the West system is getting pricey these days anyway.


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PostPosted: Thu Mar 29, 2012 11:16 am 
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I believe polyester hardens harder than epoxy. I know it is harder when the two are heated up to say, the temperature of hot sand. With that temp. the epoxy becomes easier to abrade. Some boat manufacturers who build with epoxy will not use it on decks because it gets soft in hot sun. Dynel cloth is said to be more abrasion resistant as well. Saturate the cloth and squeegee out the excess resin. You don't want glass floating in resin, that's weak and poor work. More glass with just enough resin= greater wear resistance.


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PostPosted: Thu Mar 29, 2012 1:06 pm 
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I noticed another advantage of polyester is that you can add glass layers before it is completely cured without needing to sand in between.


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PostPosted: Thu Mar 29, 2012 3:27 pm 
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I am in the finishing stages on a bottom job on my 18 and I went with polyester, it was a lot cheaper. I only added one layer of cloth cause thou, because orig resin has not worn down to the glass, only the gel coat was worn . I figured some glass would add to the beach endurance, as the boat is 30 years old. West systems 404 works good with resin if you need it thickened to patch holes on the horizontal.

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PostPosted: Fri Mar 30, 2012 9:10 pm 
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You definitely want to go with the polyester as that is what the boat is made of. If you use epoxy it is incompatible with the polyester and you will not get good adhesion. I am really surprised that your first bottom job stuck on at all. I am not surprised that you are replacing it after so few years. I have an '83 that I have owned for about 8 years and I pull it up onto a concrete ramp and the gel coat has not been worn through yet.


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PostPosted: Sat Mar 31, 2012 5:13 am 
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mdgann wrote:
You definitely want to go with the polyester as that is what the boat is made of. If you use epoxy it is incompatible with the polyester and you will not get good adhesion.

Actually, epoxy sticks quite well to polyester (and almost everything else). It's the other way around - polyester doesn't adhere well to epoxy, especially if the amine blush hasn't been removed.

Use epoxy where you need the extra adhesion, strength and where it will be protected from UV exposure and heat. It's good for making repairs inside the boat where you can't prepare the surface properly due to access limitations.

Use polyester (or vinylester) where you can provide a properly prepared surface and you don't need the extra strength (and $) of epoxy. Use it for exterior repairs where you plan to spray gel coat over repairs.

Remember that when making repairs with fiber reinforcements (whether they be glass, aramid or carbon), that the optimum ratio of fiber/resin is about 75/25 by weight (it's hard to achieve that with a hand layup, but it's a goal). Resin is weak, fibers are strong.


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PostPosted: Sat Mar 31, 2012 9:38 pm 
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Mbounds wrote: "Actually, epoxy sticks quite well to polyester (and almost everything else). It's the other way around - polyester doesn't adhere well to epoxy, especially if the amine blush hasn't been removed."

Not sure I understand this. Whether epoxy sticks to poly or poly doesn't stick to epoxy. I didn't think it would matter which was on top. Just that the two were not compatible. Or is it that poly doesn't stick to anything very well? Thanks


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PostPosted: Sun Apr 01, 2012 5:15 am 
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mdgann wrote:
Mbounds wrote: "Actually, epoxy sticks quite well to polyester (and almost everything else). It's the other way around - polyester doesn't adhere well to epoxy, especially if the amine blush hasn't been removed."

Not sure I understand this. Whether epoxy sticks to poly or poly doesn't stick to epoxy. I didn't think it would matter which was on top. Just that the two were not compatible. Or is it that poly doesn't stick to anything very well? Thanks

You have to remember that they start out as liquids and end up as solids, so the order does matter. There are also two types of adhesion - mechanical and chemical.

Polyester sticks best chemically to other polyester, otherwise, the surface needs to be roughened up to provide good mechanical attachment. By rough, I mean sanded with 80 grit. The nice thing about polyester is that you can put a new layer on top of another layer that's still "green" - not fully cured - for an even better chemical bond.

Epoxy develops strong chemical bonds with almost everything. But when it cures (especially in the presence of humidity/moisture), it will develop an amine blush - a thin waxy film that must be removed before any further coating is done. Removal is performed with soap and water, then light sanding to provide mechanical attachment.

If epoxy is properly prepared (amine blush removed, surface roughened), then poly will mechanically stick to it, just not as well as it would sticking to itself (chemical + mechanical).


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PostPosted: Sun Apr 01, 2012 9:20 am 
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MBounds wrote:
the surface needs to be roughened up to provide good mechanical attachment


This is technically not quite correct. Mechanical interlocking was been long rejected as a major factor in adhesion theory. What sanding does is:

1. Increases the contact area between 2 substances
2. Creates new fresh sites for both mechanical and chemical bonds

One practical tip is to sand within a relatively short time before applying the resin, say within an hour. Sanding on one weekend and glassing the following is not the best idea.

After sanding, you can vacuum it, but keep it clean. Some advice cleaning with acetone, but I would use it before sanding.

Jack

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PostPosted: Thu Apr 05, 2012 10:03 am 
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Well I had somewhat less than stellar results with my foray into polyester resin. I don't claim to be an expert here but I have worked with polyester in my youth fixing surfboards. I followed the instructions as well as several tips mentioned here. I sanded with 80 grit and cleaned with acetone. I laid up two layers with 9 oz fiberglass tape, the second while the first layer was still green. It seemed to cure fairly well and I let it harden for two days before sanding. All-in-all it seemed pretty solid. While feathering the edges with the sander I noticed the resin would chip here and there so I decided to stress it a little to see what kind of adhesion I had. I poked a glaziers knife under the edge where the chip appeared and lifted a bit. The new glass started to de-laminate so I pushed a little harder and, before I knew it, the entire repair lifted up as a single, 90 inch long mini-mold of the hull. Now I've done two bottom jobs with epoxy and never had them lift like this. I must say though that I didn't sand the hulls down to the bare fiberglass so most of the adhesion area was over the existing factory gel coat which I assume to be epoxy. Based on this I would say polyester over epoxy is rather poor choice. Fortunately it was on sale and I'm only out 17 bucks and a little sweat. I can also confirm that polyester is a lot harder to sand and finish than epoxy.


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PostPosted: Thu Apr 05, 2012 11:14 am 
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gel coat is polyester resin not epoxy. IF you did not clean the sanded surface with acetone or compressed air it wont stick good.

Ive heard that gel coat sticks to poly resin and poly resin don't stick to gel coat.
It failed because you put resin over gel coat.

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PostPosted: Thu Apr 05, 2012 11:21 am 
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sunvista wrote:
Well I had somewhat less than stellar results with my foray into polyester resin.


Gelcoat is polyester based, not epoxy. There is no such thing as epoxy gelcoat...that would be paint. Hobie hulls are not built using epoxy anywhere. They only use polyester resin.

This sounds like you did an insufficient job of prepping the repair surface. It needs to be well sanded and cleaned with acetone. If the hulls have been recently polished/waxed, I would be extra thorough to be sure you've gotten all of the wax/comtaminant off of the surface. If the hulls are dirty, you probably want to wash them before you do any sanding. That way you don't sand contamination into the hull surface. Once you've prepped the surface, DON't touch it with your bare hands or allow any other contamination.

I would not use epoxy for a bottom job. If you ever have to repeat the repair in the future, you would have problems if you then used polyester resin, or if you attempted to gelcoat the hulls.


jmecky wrote:
Ive heard that gel coat sticks to poly resin and poly resin don't stick to gel coat.
It failed because you put resin over gel coat.


Polyester resin will bond just fine to gelcoat as long as you properly prep the surface.

Although having said that, there would be no reason to put polyester resin over gelcoat (for a bottom job). You only need to use polyester resin and fiberglass cloth if the hull is worn so badly that you've worn through the gelcoat and into the existing fiberglass. If you haven't worn the hulls down through the gelcoat, then just fill in the scratches with formula 27 and re- gelcoat...no need for fiberglass and resin.

sm


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PostPosted: Thu Apr 05, 2012 5:57 pm 
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Geez, your problem could be lotsa things
Old resin bought on sale, it has a shelf life. Frequently 'sales' come at end of life
Old MEKP hardener. Hardener has a shelf life considerably shorter than resin. Only about 6 months. Usually 'sale' resin comes with funky MEKP.
Poor mixing ratio for a given temperature.
Insufficiently stirred. More being better.
Poor prep.
Wax on the surface from old resin.
Acetone is a strong solvent, it will dissolve synthetic fabrics and put contamination on your work. Cotton rags are best for Acetone.
Bad Luck


Last edited by ChrisD on Sun Apr 08, 2012 8:39 am, edited 1 time in total.

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