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Old 27-11-2018, 18:17   #1
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Polyester resin shrinkage. Is this a real concern?

Over the last 6 weeks or so I have stalled a bit on my deck re-core project as home obligations had pulled me away from boat obligations. During this down time I had some time to think and re-think about this project

At the time I stopped the boat work, I was ready to start putting down new glass on my newly laid Coosa cored deck. I thought I had read enough about the differences of using epoxy vs. polyester resin and several well respected contributors to this and other forums eased my mind and I decided to go ahead and use polyester resin to replace the original polyester deck skin. "Like with like" was what they said. These are the reasons I decided to go with polyester.

1. They say the bond strength is suffcient for this kind of repair.
2. Being less waterproof than epoxy is not an issue on the deck.
3. The entire laminate can be laid up without sanding.
4. Cheaper and quicker polyester fairing compounds.
5. Substantial difference in the cost of epoxy over polyester.
6. I will be painting the decks with Awlgrip and priming with 545 primer so either resin will be compatible with the topcoat.

I thought I had heard all the pros and cons of epoxy and polyester until I read this from the Epoxyworks website about how much polyester resin shrinks when it cures.

https://epoxyworks.com/index.php/wes...s-boat-repair/

“Epoxy shrinks less
These were fairly small repair specimens compared to repairs often undertaken in the field. Some experts believe that the larger the repair, the more important it is to use epoxy. Their concern is related to the shrinkage that occurs in polyester resin. In a small repair, shrinkage is fairly insignificant. As the size of the repair increases, especially when a large section of laminate is being replaced, the shrinkage causes the repair to be stressed before the structure sees any working loads.
Forbes Aird in his book Fiberglass and Composite Materials provides a good description of what can happen in a bond line when polyester is used as an adhesive. He says that polyester has a volumetric shrinkage of about 7% during cure. Because of this, a bond line is subjected to significant stress which will occupy or use up a substantial fraction of the resin’s bond strength even before subjected to any working loads. So, the 70.5% repair effectiveness for polyester identified in the testing becomes something less when used for large-scale repairs.”


This concerns me a bit as my side deck will have a bonded edge of 30 feet on the outboard edge alone! Now, I am no engineer and I don’t know how “volumetric shrinkage” is calculated but in my simple mind 7% of 30 feet is about 25 inches. That seems like a lot of tension on that long bond line without ever taking into account the tension the rig will put on the deck and the flexing of the hull in rough sea’s.

I can’t help but think that epoxy is the better choice here because of this point alone but I really respect the opinions of the professionals here and elsewhere and they all seem to use polyester for this kind of repair for their customers.

I am having a hard time reconciling this "new to me" information with the advice of the pro's here.

Is this a real concern for my large decks?
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Old 27-11-2018, 21:00   #2
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Re: Polyester resin shrinkage. Is this a real concern?

I don't think your job is a "repair" as you are relaying the complete deck. I'd be a little concerned with the hull deck join as you are using new glass to adhere to f/g that was laid ? years ago.

When you are laying glass onto existing glass it is better to do so within 7 days so you get a chemical join as well as a mechanical join. In your case you should sand the existing glass (hull) to give the newly laid glass a "key" to adhere too

Since the deck has been laid up well and truly after the hull has cured maybe you should bolt the deck to the hull. ( have done that to mine)
http://wavetrain.net/boats-a-gear/31...ll-deck-joints

I'm not surprised a manufacturer/seller of epoxy knocks the opposition : in this case polyester!

I think you are reading too much into this and worrying un-necessarily.

I'm quite sure I can find articles written by experts which will allay your fears.


Clive
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Old 27-11-2018, 21:09   #3
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Re: Polyester resin shrinkage. Is this a real concern?

There are 565,444,373 fiberglass shops doing deck repairs on boats. The vast majority of them use Polyester, and do a fine job. I'd not worry about it. It's easy to overthink things. Be sure you have a good overlap, proper mixing, and a good scarf. Do the job properly, without shortcuts, and you'll have a fine deck.

Perhaps get a little advice from a glass guy, who does these sorts of repairs. He'll show you a trick or two.

You'll get many opinions here on CF, and many from those who've never done any glassing, and they'll all swear they know exactly what they're talking about.

I've done a fair bit of glassing, but not much with poly. When I did the deck on my old boat, I used epoxy, because I chose to use cedar for the core. I wouldn't have any qualms using poly otherwise. Most all other glassing on that boat was with poly. Engine mounts, autopilot mounts, chainplates...

Epoxy is "stronger" than poly, but most use poly.

I agree with Clive, you're overthinking it.

If you're rebuilding the entire deck, including the toerail, by all means, bolt it down. If it's already bolted, and you're glassing up to the solid bits, then I wouldn't bolt it down. I'd just let the original bolts take the load.

Cheers.
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Old 27-11-2018, 21:12   #4
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Re: Polyester resin shrinkage. Is this a real concern?

Grit


I agree 100% with your comments. Too may expert fiber-glassers on these threads!
And I'm sure Wind River is worrying too much.


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Old 27-11-2018, 21:20   #5
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Re: Polyester resin shrinkage. Is this a real concern?

Clive. Is that a Glass "Roberts 43" you're building? Lovely boats. Pilothouse, centre cockpit, or aft?

Clive, I think that deck's already bolted under the teak cap rail. I think he's just rebuilding the cored part of the deck.

At least that's my take from looking at the picture.

Cheers.
Paul.
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Old 27-11-2018, 21:37   #6
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Re: Polyester resin shrinkage. Is this a real concern?

How much more would epoxy cost? Is the peace of mind worth it?
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Old 27-11-2018, 21:49   #7
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Re: Polyester resin shrinkage. Is this a real concern?

Quote:
Originally Posted by GRIT View Post
Clive. Is that a Glass "Roberts 43" you're building? Lovely boats. Pilothouse, centre cockpit, or aft?

Clive, I think that deck's already bolted under the teak cap rail. I think he's just rebuilding the cored part of the deck.

At least that's my take from looking at the picture.

Cheers.
Paul.

Paul

It has an Airex cored hull and Divinycel cored deck and is center cockpit.

Right now I am wiring up the winch, inverter, water pump, bilge pumps and taking power to the switchboard. But I also have made some progress on the steering.

I have an album on the yacht on Flickr here.

https://www.flickr.com/photos/440710...57631756633356


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Old 27-11-2018, 23:02   #8
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Re: Polyester resin shrinkage. Is this a real concern?

Epoxy resin shrinks too. How much depends on who you ask, but there is no point in discussing one without the other.


I've used both, though I never quantified results. Ease of use depends on methods. Epoxy seems to have slightly greater bond strength, but either can have more than enough strength, if the project is designed properly. Polyester stinks. I probably would have used more polyester if I had known what I know now. Epoxy when very high strength and perfect secondary bonds are needed or if I'm working indoors. Polyester for a large re-deck.






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Old 27-11-2018, 23:56   #9
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Re: Polyester resin shrinkage. Is this a real concern?

Polyester is fine, and is the standard of the industry.

As examples, following are a few before/after pictures of some of many repairs/restoration from over the years, all with polyester and pvc foam, coosaboard, and/or treated plywood in various combinations.

'Shrinkage' is less of a problem over large spans than small.

1 & 2 - 22' 80 mph bass boat, coosa/polyester

3 & 4 - 33' Egg Harbor, pvc/polyester (about 10 x 6 crowned rear deck)

5 - 24' balsa-cored Seabird, balsa core removed, replaced with pvc foam/polyester, 3 1/2' hull extension fabricated of pvc polyester and attached with polyester

6, 7, 8 & 9 - 22' Wellcraft transom/stringer/deck replacement with 3/4" treated plywood, coosaboard, pvc and polyester
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Old 28-11-2018, 04:43   #10
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Re: Polyester resin shrinkage. Is this a real concern?

Clive. I'm at the wireing stage as well. Lots of fun. I'll look at your photos; sorry I don't have a website.

I will PM in future, so as not to derail this thread.


Cheers.
Paul.

PS. I just looked at your photos. What a beautiful job you've done. Our interior is not as finished as yours. The bridgedeck is missing the drawers and stove, and the hull interiors are just rough. We're corecell in the hulls and deck, and honeycomb in the furniture. It's all epoxy infused.

Cheers, again.
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Old 28-11-2018, 05:33   #11
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Re: Polyester resin shrinkage. Is this a real concern?

Small hijack, more polyester thoughts.

I'm interested in the speed of assembly of polyester for non structural fabrication.

I'm talking about cabinets, waste tanks, non structural bulkheads.

I'm deciding between 1/4" marine ply cabinetry and foam core poly cabinetry. Trying to understand the speed of working with poly as compared to epoxy.

Anyone have input on this as well as input regarding tabbing these cabinets to an epoxy hull?

One thing is my glass is all uni/biax and triax without mat. Can I even use polyester with this glass? They say you need mat to bulk up the poly.

Is there a good set of information somewhere on how to best work with poly? Tons of it on epoxy and can't seem to find anything on poly.
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Old 28-11-2018, 06:15   #12
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Re: Polyester resin shrinkage. Is this a real concern?

Iv'e been using both epoxy and polyester professionally for over 40 years and one thing i have noticed is that amateurs tend to overwhelmingly use epoxy for everything while pros tend to use what is appropriate for the job at hand.It may be epoxy, it may be poly or it may be vinylester but the choice is based on experience and knowledge. The reason for this is that epoxy has been heavily marketed to amateurs in just about every retail publication while polyester and vinylester have only ever been marketed to professionals so the only opposing views an amateur will get is on forums like this where they will also continue to get marketed to by amateurs who have been marketed to and don't even realize it. If you are an boat owner and need to keep a kit of something on hand for small projects then epoxy is a no brainer as it will cover just about any project but when doing a bigger project where you need larger quantities then you may want to listen to the pros. Either will do a fine job and quite frankly we tend to use VE rather than PE for most larger projects unless there is wood involved but i agree with the others that you are overthinking this as the stresses in most parts of the boat are very low and even PE is way stronger than needed. I have yet to see a proper scarf joint fail with any resin system.

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Old 28-11-2018, 07:18   #13
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Re: Polyester resin shrinkage. Is this a real concern?

Chotu.

Poly is generally not recommended for your project, since it reputedly doesn't stick well to epoxy. I've heard with good abrasion, that it will do the job, but I'd not bond tanks in with it.

Due to it's lower bonding strength, the glass manufacturers always recommend using glass with mat. Not the stuff you're accustomed to using. The mat ensures that there's more strength in the poly, right at the interface of new/old. Without the mat, the poly bond is less strong.

I wouldn't use it for that.

I would, however, build the furniture and tanks in poly, then after sufficient cure (epoxy, supposedly, doesn't like fresh poly), tab it to the hulls with epoxy. This will help with your epoxy sensitivity.

Cheers.
Paul.
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Old 28-11-2018, 10:19   #14
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Re: Polyester resin shrinkage. Is this a real concern?

I'm in the camp that believes your concern about shrinkage is unfounded. Yes there is some but unlikely enough to be an issue. The deck hull joint would be my concern.
Insure the old glass areas of your joint are sanded with lots of tooth.
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Old 28-11-2018, 10:21   #15
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Re: Polyester resin shrinkage. Is this a real concern?

Quote:
Originally Posted by clockwork orange View Post
Iv'e been using both epoxy and polyester professionally for over 40 years and one thing i have noticed is that amateurs tend to overwhelmingly use epoxy for everything while pros tend to use what is appropriate for the job at hand.It may be epoxy, it may be poly or it may be vinylester but the choice is based on experience and knowledge. The reason for this is that epoxy has been heavily marketed to amateurs in just about every retail publication while polyester and vinylester have only ever been marketed to professionals so the only opposing views an amateur will get is on forums like this where they will also continue to get marketed to by amateurs who have been marketed to and don't even realize it. If you are an boat owner and need to keep a kit of something on hand for small projects then epoxy is a no brainer as it will cover just about any project but when doing a bigger project where you need larger quantities then you may want to listen to the pros. Either will do a fine job and quite frankly we tend to use VE rather than PE for most larger projects unless there is wood involved but i agree with the others that you are overthinking this as the stresses in most parts of the boat are very low and even PE is way stronger than needed. I have yet to see a proper scarf joint fail with any resin system.

Steve.
Thanks everyone for the input.

Steve (and others) - I think you really said what I felt in my gut was correct. I had referred to several deck recore thread here on this forum where Miraret and other professionals were giving advise to another DIY member with a similar project. Not to leave out any of the reputable professional members of this forum (thank you all and you know who you are), but I have followed Minaret's work and advise here for quite a while have grown to trust his recommendations, but all of you pro's seem to agree on most things fiberglass.

In my down time I kept reading and seemed to find more of the #NeverPoly and #NotMyResin kind of posters arguing the benefits of epoxy to the pro's and the pro's just were not taking the bait. As Steve points out, many of these posters were amatuer DIY guy's like myself and probably want to err on the side of the most tenacious bond they could get, maybe, just in case they didn't really understand what the mechanics were of what they were doing as they don't have as much experience with it.

"Experience is something you get only after you need it"

There is no argument that epoxy adheres better than polyester and that is my concern..... making sure the new glass never detaches from the old glass. This is a huge project for a pro, let alone a DIY'er like me. I don't want to ever have to redo this. I have the 12:1 scarf, I have sanded the surface with 36 grit sandpaper and it will be cleaned with Acetone before the new glass is applied. Another pro on this forum (JohnMardall) has suggested adding a series of holes in the old laminate to add a "geometric bond" as well. I really like this idea, thanks John. (see his recommendation here: Fiberglassing my newly cored deck. )

It seems idle minds create their own demons and that is what happened here with me. I never read any discussion of this shrinkage issue before and when I read this article, the alarms went off. I thought I was pretty well read on the subject until that.

I will do it like the pro's and use Polyester.

Thanks everyone.
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