stern hull gel coat, re-coat

Long time lurker, first post. I recently became an owner of an oldie Necky Arluk III (3). The stern bottom is bare and looking to add some gel coat for protection. I’ve been looking for details and keep stumbling upon the word “Polyester Resin” and “hydrolysis”. Were 20+ year old kayaks made with polyester resin at any point or is it safe to assume that they used Epoxy Resin. Doing my due diligence and don’t want to mess anything up. Also, if I were to make repairs I would definitely use Epoxy Resin.

Any help is appreciated and thanks in advance for this awesome community.

I strongly suspect that the boat was built using either epoxy or vinylester resin, but I don’t know that for sure. Polyester resins such as gel coat may not cure properly. Epoxy will cure over either polyester or vinylester resin. I would be inclined to use epoxy to refurbish the hull bottom, then paint is with a marine polyurethane paint such as Interlux Brightside or Pettit EZ-Poxy.

My guess is that the finish is polyester gel coat if the boat is fiberglass.

I think my wording in the question may have been confusing. I may have this all backwards but older kayaks may have used polyester resin to bond the fiberglass together thus creating the kayak and I have been reading that hydrolysis can occur when polyester resin was used. If I improperly apply gel coat finish and water cannot escape then it will de-laminate the fiberglass mats. My question is did they ever use the technique of applying polyester resin to mats to create a kayak or was vinylester more common?

Polyester resins are distinctly inferior to epoxy and vinylester resins for a number of reasons. Necky composite kayaks were fairly decent quality so I doubt they used polyester resin to build their boats. As for hydrolysis, I think that is a process much more likely to involve a boat that is left in the water for protracted periods of time, much less likely to affect a canoe or kayak, most of which spend a relatively minor portion of their life in the water, and have plenty of time to dry out between immersions.

Gel coat applied as the first layer inside a female mold is not the same as gel coat applied on the outside of a constructed boat. I would stop worrying about hydrolysis and worry more about whether polyester gel coat will cure if you apply it to your boat. I have heard some unpleasant stories from individuals who tried to re-gel coat boats and wound up with a sticky, gooey mess that had to be laboriously scraped and sanded off the hull.

As far as I know, all gelcoat is polyester-based. Regardless, there is no problem with gelcoat bonding to vinylester laminate and it will even bond to epoxy just fine if the resin is fully cured and cleaned to remove any blush.

The easiest repair method is to get “finish” gelcoat paste, which is thick enough that it will not run when you’re working on sloping or vertical surfaces, such as at the bow and stern. Finish gelcoat will also cure without having to cover it (laminating gelcoat won’t). It’s also a bit stronger than laminating gelcoat, according to one manufacturer I contacted.

If you decide to use epoxy, there are some caveats. If the mixed epoxy is amber, it’s going to have a yellow tint if you add white color to it, so it won’t match the existing gelcoat perfectly. Epoxy is runny, which means you need to add a considerable amount of thickener to it in order to keep this in place. I’ll do this on paddle tips, but there’s no way I’d do it for a large area on a hull. Finish gelcoat is much easier and faster.

The only ways that gelcoat won’t cure are:

  • If it’s just too old (especially if it’s been opened previously). You can test this before applying it by mixing a small amount.
  • You don’t add the proper amount of catalyst and/or don’t mix it thoroughly. Read the directions!
  • You apply it over an incompatible surface like uncured or blushed epoxy. Don’t rush the process; epoxy takes time to fully cure and any amine blush must be washed off.

I have heard multiple stories that I judged to be reputable from individuals who tried to apply polyester gel coat, and other polyester resins, over very well-cured epoxy, for example boats that were years old, and had failures to cure. I have also heard stories from people who did the same who were successful.

Epoxies even when fully cured have an abundance of amine excess in the cured resin. I cannot say I understand the chemistry fully, but I have read that under some conditions, fresh polyester resin applied over cured epoxy can cause sufficient amines to leech from the surface of the epoxy to interfere with cure of the polyester. West Systems states emphatically that polyester gel coat can be applied over their cured epoxy after proper surface preparation, so this may be a phenomenon that varies with the epoxy used to construct the hull. It may also depend on the gel coat used. Evercoat is a popular brand of polyester gel coat and their instructions specifically state that it should not be applied over epoxy even if fully cured.

I’ve used the West Marine Finish Gel Coat on vinylester and epoxy kayaks over the years. I found that the gel coat in the can doesn’t go bad. The little tube of MEKP methyl ethyl ketone peroxide has a very limited life. You can pick up a fresh small tube of MEKP every year relatively inexpensively, and keep your can of gelcoat for years. The only time the stuff didn’t cure for me is when I tried to use old MEKP. When the leftover doesn’t cure in the mixing container, nor on the mixing stick, you know it had nothing to do with the composite material where you applied it.
It’s easy enough to test with just a drop. Use a teaspoon of gelcoat with 3 drops of MEKP, mix, and apply a drop on the surface in question. When your mix hardens, your drop should be hardened.

Thanks for the tip, CapeFear. I’ll weigh my options between using gel coat to add that sacrificial layer or another option from an old thread I found by pbblanc to simply paint it with marine paint and just forget about it. Luckily, it still has all the fiberglass resin left on it and the white gel coat is the only part that “sanded” off over 20 years of use.

If there is any texture to the exposed fabric, you could put a thin layer of epoxy one it to completely fill the weave and impregnate any fibers that have started to abrade with resin before you paint. That will result in a smoother surface.

If you feel that there has been any damage to the outer layer of cloth, you could bond one layer of 4 ounce/square yard S fiberglass over the area. That would add a good bit of abrasion resistance and strength for little weight gain.

If the exposed "glass is very smooth to the touch, I would probably just paint over it.

@pblanc said:
Evercoat is a popular brand of polyester gel coat and their instructions specifically state that it should not be applied over epoxy even if fully cured.

FWIW, I’ve used Evercoat gelcoat over epoxy many times, without any problems. I suspect that their warning is because they probably just don’t want to deal with complaints from users who’ve bad experiences due to applying it to under-cured or blushed epoxy. I can’t say that I blame them.

Here is an extract taken from the general FAQ page for System Three, a pretty well known epoxy manufacturer:

"Can I put polyester gelcoat over cured epoxy?

In general, polyester resins won’t cure properly or bond well to epoxy resin products without a “tie-coat” barrier resin in between. System Three SB-112 resin system can be used as a tie coat in between epoxy laminating or coating resins, and polyester laminating or gelcoat resins. Using this resin allows you to gelcoat an epoxy-built or epoxy-repaired boat."

Now, I have never had a failure of polyester gel coat to cure when applied over cured epoxy. But I have never tried to do so. All of the tales of woe that I have read about on the internet were second hand so I have no way of knowing whether the cure failure might have been due to other factors such as poor surface prep or failure to provide an adequate air barrier. I am also aware that individuals have successfully applied polyester gel coat over cured epoxy on many occasions.

I do know that if you frequent various boat building forums you will find the admonition against using polyester gel coat over epoxy frequently repeated. And I have also heard this same advice from a well-known individual who was intimately involved with composite canoe manufacture.

Is the notion that polyester resins won’t cure properly when applied to cured epoxy all a myth? Maybe so, but it seems to me that if well known manufacturers of polyester gel coat, well known manufacturers of epoxy, and experienced boat builders all advise against the practice, there is probably some basis somewhere for that advice. My guess is that certain conditions, involving certain epoxies and certain polyester gel coats may result in a failure to cure.

@pblanc said:
My guess is that certain conditions, involving certain epoxies and certain polyester gel coats may result in a failure to cure.
I would say that’s correct. I think the biggest issue is that there’s no way to accurately compare DIY repair jobs and their results, because conditions of the boats, the repair materials and the prep can vary so much. All I know for certain is what has worked for me in the past.