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glhotka · 22-02-2017, 05:04

Question on Fiberglass. I have a 51' Formosa and have some soft spots in the deck (There is a bit of give there if I bounce up and down standing on the deck. For the most part, it is at the widest part of the deck just forward of the coach-house and runs about 4 feet from coach house forward.

As I am replacing core in the deck, the question of Epoxy vs. Polyester is coming up due to the significant difference in price between the 2. So if I cut out the outer skin, expose and replace the core with new hardwood, can I put on Polyester Resin, matt/woven?
Or since this needs to attach to the ground raw fiberglass edges of the cut opening, does it need to be epoxy.

There seems to be confusing suggestions when searching the interweb for the recommendation. I think I may have 20% less adherence strength with polyester to existing fiberglass than using epoxy, but I am not sure if 20% should be enough to sway the decision toward epoxy.

Here is how I am doing the repairs if this would impact the answer. I have removed all the teak off the deck already and removed the screws. I have done a "tap test" and found the spots that sound delaminated (thud vs solid sound) and have used a circular saw set shallow to cut through the upper deck skin. I started with a section about 30" wide by about 5 feet long. I removed all the core and replaced it with mahogany hardwood boards (that is what was in there when I opened it up). I am using small 5" by 12" strips that I epoxied to the lower skin with thickened epoxy, then when cured, used a thickened epoxy in and around all those boards to form one solid core. I then ground down the existing edges of my outer skin cut (about 3" wide) so that each layer of fiberglass matt would be slightly larger than the last and overlap and integrate into the existing deck. I put one layer of glass down ( the cross strandFirscross-strand matt, then one layer of 24 oz woven roving. I need to add a few more layers to get the spot thickened back up to about 1/8" of glass, but I stopped and am pondering this question of resin type. This one opening has already used more than 1.5 gallons of resin and the hardener that goes with it.
Given the fact that in this spot I have already put a layer of epoxy resin and glass down, does that change the answer on the material to use? And what about the next section I will be removing and repairing?

Love to get some feedback from the crew of "been there done that" here.

drakes · 22-02-2017, 05:10

my understanding is you don't put polyester on top of epoxy

once you start with epoxy i would finish with it. i don't understand all the technical details. sure someone else can jump in but it has to do with chemical properties or adhesion.

Benz · 22-02-2017, 05:35

Having got so far with epoxy, you really should continue with that--polyester won't really bind to it so well. However, since it's just a skin to thicken things up, you don't really need bombproof adhesion--it's not like something's gonna try and peel up your laminate.

Look into painting a primer onto the epoxy, and then using vinylester over that. I have used a special primer called Duratek as a tie coat between epoxy and successive layers of polyester material. Again, given that it will lie flat and not have a peeling-off pull, I think it's a good solution. And make sure that whatever you do, you sand the epoxy thoroughly before putting any type of material over it.

For repairs in other spots, I recommend using all vinylester start to finish.
I'm doing some fiberglass projects myself this winter--there might be details relevant to you on my blog:
zartmancruising.com

Best,
Ben

toddedger · 22-02-2017, 06:46

Make sure you post cure your epoxy with heat (120°) or it will continue to shrink long after you are finished with the repair.

Terra Nova · 22-02-2017, 07:52

Quote:

Originally Posted by toddedger

Make sure you post cure your epoxy with heat (120°) or it will continue to shrink long after you are finished with the repair.

Bull.

rh81 · 22-02-2017, 08:15

Quote:

Make sure you post cure your epoxy with heat (120°) or it will continue to shrink long after you are finished with the repair.

Done many things in Epoxy and never heard about that........
Post curings adds strenght to the laminate.
If you mean 120°C than you mess up the whole thing. Most of the Epoxis are post cured at around 60/80°C

leftbrainstuff · 22-02-2017, 08:26

For repairs and hand layup the difference in ultimate properties between epoxy and polyester is not significant.

Epoxy benefits from a strong primary bond when you complete the layup and wetout in one go.

With polyester you don't lose strength if you have time between layups. We chose polyester for our hard dodger windscreen layup which has taken months due to relocating half way through the job.

With polyester resin you dont need to sand between layups like you do with epoxy. Makes the job cleaner and quicker.

For our custom helm I used epoxy. In hindsight I should have used polyester as finishing the job in the cold pacific northwest has been a pain in the butt. I have had some epoxy not cure as I failed to provide enough heat during cure. This rework is a PITA.

All my resin decisions have been primarily focussed on ease of completing the job. UV resistance, ultimate strength and other physical properties of different resins is not an issue. They are all good enough for most of our boat projects.

Note that epoxy adheres much better to polyester than the reverse. If in doubt use epoxy for your repair. The bond between new and old layup is the critical non functional requirement for your repair.

I would use epoxy and design the repair to be completed in one go.

rh81 · 22-02-2017, 08:31

If you want to avoid sanding between the layups you can use Peel Ply. Just peel it away and you are ready for the next layups

Colin A · 22-02-2017, 08:32

In general Polyester would be fine for those repairs. As mentioned the only real issue would be the interface with epoxy repairs. There are ways around it but the bond would likely be the least strong in the repair.
WEST SYSTEM | Projects | Fiberglass Boat Repair and Restoration - Polyester over epoxy

toddedger · 22-02-2017, 10:27

Sorry guys I've been in the industry working with epoxy for over 30 years. The 120° came directly from Gougeon brothers. Your epoxy will cure, but as soon as the sun shines on it and heats it up it will continue to shrink. Sometimes I wonder why I even bother to help you guys out.

rpjn59 · 22-02-2017, 11:22

I thought the rule of thumb when working with polyester was that if successive layer are applied to a tacky to touch preceding layer, no sanding was required. But if the preceding has cured for a period of time, not tacky to the touch, sanding was necessary, followers by acetone wipe and then tack clothed. Correct me if Im wrong.

**neilpride** · 22-02-2017, 11:25

My vote for Poly, and you can always barrier coat with epoxy...

ArmyDaveNY · 22-02-2017, 11:36

In addition to the information you have received, I have two suggestions. First, call the manufacturers of the epoxy you have used and the polyester you are considering. I have found that most manufacturers are happy to help and often have very useful insights.

Second, make a test bond (NOT IN THE BOAT) and see how that works. Test it to destruction and see if you are satisfied.

Given the time and money you are investing, it will probably be well worth it to do a test. If there is a possibility of shrinkage or other problems, I would rather find out before I spend a lot of time and money. I wish you the best of luck.