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  #1  
Old 06-29-2008
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Deck Recore - Polyester Resin?

I am doing a deck recore on a couple of sections of my Ericson 29 and am ready to put things back together. I am going at it from the top and am going to put new balsa core in and put the top skin which I saved back on.

Most of the posts I have seen all appear to refer to using Epoxy to do all this but why can't I just use Polyester resin. It seems a lot easier to work with and while not as strong as Epoxy is what the boat was built with. Is there any really compelling reason I must use Epoxy? When I need more Polyester it is really easy to mix and is a lot cheaper.

If good enough for building the boat I figure it must be good to repair.
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I think you're right. I rebuilt my O'Day 25 decks 5 years ago with Polyester and I cannot think of a reason to do that work with epoxy. It's held up fine. There has been some issues with paint adhesion. Be sure you wash the amine blush off and the wax off before you paint or you will suffer massive paint loss. I was less than thorough in this regard and have suffered greatly as a result. Epoxy might be better in places where there are stress cracks but there as well, you might be just as well off with more layers of fiberglass cloth than what you took out... Good luck!
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Old 06-29-2008
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It's not how...

It's not how strong it bonds during lay up and the initial build but how poorly polyester resin bonds secondarily to an older already cured hull.

Polyester resins have very poor secondary bonding characteristics which is why most everyone and boat yards/builders use epoxy for repair work. Polyester resins also tend to kick way to fast for use in large repairs like deck re-cores where longer open working times are required..

There is nothing easier to work with than epoxy resins especially with the metered pumps..

How many drops? How big was my pin that I punctured the MEKP tube with?

"Drops" of MEKP are NOT a metered size and thus are very subjective so getting a consistent polyester to hardener mix ratio is VERY difficult..

P.S. Don't forget that there are two types of polyester resin "finishing" and "laminating" resins. Laminating resins will never fully dry and will always be tacky!
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Last edited by Maine Sail; 06-29-2008 at 12:31 PM.
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Old 06-29-2008
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Epoxy for the reasons Halekai mentioned. Also, the fumes from polyester resins are yet one more reason to avoid them. As well, if you purchase the right brands of epoxy, you will not have to deal with amine blush. All of these reasons make epoxy worth the extra expense.
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For what it is worth I decided to use both. I am building up the bottom skin and repairing some holes made when I was removing core with glass and polyester. I did a little of this already and it appears to be bonding and curing well. I will then epoxy the new core in place and epoxy the top skin back on.

The one thing that confuses me Halekai is the issue you pointed out with measuring drops etc. I understand polyester resin cure time can be adjusted by adding more or less drops. In the end it does not matter how many drops you put in, it will only affect the cure time. This is what I meant about polyester being easy to work with. Epoxy you have to get ratios exactly right which is admittedly easy with the West system pumps.

The smell of the polyester is pretty nasty but I use an organic filter which negates that. I use nitrile gloves for both but worry about epoxy getting on my skin is much higher.

Thanks for the input.
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Quote:
Originally Posted by maccauley123 View Post
The one thing that confuses me Halekai is the issue you pointed out with measuring drops etc. I understand polyester resin cure time can be adjusted by adding more or less drops. In the end it does not matter how many drops you put in, it will only affect the cure time. This is what I meant about polyester being easy to work with.

Thanks for the input.

NO, no, no!! I don't know where you got that idea but it is not true at all. Polyester resins are very temper mental and require the right amount of MEKP for the temp and humidity. If you mess up the temp/RH/MEKP ratios you can make a real mess!!

This is directly from the Evercoat web site they are one of the largest resin suppliers to the chandleries:

"Hardeners for polyester resin, often referred to as catalyst, come in small plastic tubes or bottles with graduated measurements marked on them. Hardeners are measured in drops or fractions of teaspoons for most lay-up or repair jobs. Consult manufacturer’s instructions. Do not over-catalyze or under-catalyze -- you may ruin the resin and have to start again. This will only cause further problems.

Polyester resins have a limited shelf life of one year. Therefore, be sure to purchase fresh resin and try to use it within six months from the time you purchased it."
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Halekai, I have seen that in numerous places. Don Casey in This Old Boat talks about adding more or less catalyst to adjust cure times based on temp and all. Allan Vaitses in The Fiberglass Boat Repair Manual says polyester resins cure time can be adjusted on the job.

I am using Evercoat Boater's Resin and the instructions on the back give the right number of drops based on temp. It only says be careful when adding more catalyst to speed up the cure because too much heat could be generated.

I only meant that Epoxy requires exact measuring but Polyester is a little more flexible.
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Both..

Yes Casey and Vaitses make mention of adjusting cure time on the job and relative to temp but it can happen that you get too hot of a mix and temps can approach 300 degrees. It can also happen that you get an incomplete cure because, as I stated, not all "drops" are the same. I've seen folks use a 16d nail to punch a hole in a MEKP tube and others use a sewing pin. A 16 penny nail to a sewing pin is a very large variation and counting drops of non-standard sizes can potentially lead to problems.

It sounds like you have a decent handle on it but do know that you can under and over mix polyester and get poor structural results.

Usually folks over mix and it kicks off too soon leaving you with dry spots in the cloth or voids that could not be rolled out....

Also, as a poster above found out removing the wax, from the use of a finishing resin, requires more than just sand paper..

Honestly I'd rather see you use epoxy and I'm willing to let you buy it on my Hamilton account as a gesture of how strongly I feel about this..

PM me and I'll give you my info so you get my discount...
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You really should be using EPOXY RESIN for any secondary repairs. It has a lot to do with secondary bonding characteristics as pointed out previously. Also, to get a good bond between the old laminate and the new core material in a recoring project, you really want to use thickened epoxy. The strength of the bond between the core material and the laminate skin layers is crucial to providing the strength to a cored laminate.

If you use polyester resin, especially on older boats, the number of "bonding sites" for the polyester to adhere to is very low, and the resulting laminate will be relying on the much weaker adhesive strength of the polyester resin. Epoxy's adhesive strength is much higher than polyester resin.
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" I've seen folks use a 16d nail to punch a hole in a MEKP tube and others use a sewing pin. A 16 penny nail to a sewing pin is a very large variation and counting drops of non-standard sizes can potentially lead to problems."
And here I thought a drop was a drop, and the drop size was determined by physical constants like the surface tension of the fluid and the local atmospheric conditions--not by the size of the hole.
I've never seen reference to drops (measured drops of any kind, cooking, medicine, whatever) that noted the hole size as being significant. Are you certain it is? Assuming you let the drops form--not squirt the fluid out?
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