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Gelcoat is available in 2 formulas, one has wax (finishing), one does not (layering). Both need to be mixed with an agent to cause it to harden. Gelcoat that is exposed to the air stays tacky, this allows for a chemical bond to form between layers. When using gelcoat for the finishing coat of a boat repair, the common method is to use the formula with wax. As the gelcoat sets, the wax floats to the surface and forms the barrier that allows it to fully harden.

While shopping for a product at a popular web seller, I found this review for a verified purchase of TotalBoat White Gelcoat.

Was happy at the time of application on how it was ..., October 20, 2016

I had called the company to make sure I understood how to apply product and mix contents. Got the boat ready and applied the gelcoat. Was happy at the time of application on how it was going to work, but when came back next day there were spots that had not hardened and still have not. I do not understand what went wrong and will be calling the company back on what possibly went wrong. I mixed hardener the same, the surface was all prepared the same. Will have to redo the boat and not going back with this product.Source.

According to the information on the product page, this gelcoat was made with wax. I searched the site where I found the review, but couldn't locate a wax-free product. I ended up going to another source to get a wax-free product for my first layer.

Obviously I don't know for sure, but the description of "spots that had not hardened" makes me wonder if maybe they used soap and water (acetone is recommended by some) to clean the boat prior to applying the gelcoat. If so, and some soap residue remained, it may have potentially risen through the gelcoat, repelling the wax in "spots".

Assuming the person mixed the gelcoat and hardener well, what could have caused some areas not to harden?

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  • @Paparazzi I look forward to your meta post. I can't see how a question about boat repair would not be in scope.
    – James Jenkins
    Commented Nov 12, 2016 at 13:26
  • I think this looks on-topic, mostly because of the other questions we have regarding maintenance and repair of boats. Gelcoat is mentioned in some. Also, this has an answer that's clear and well-researched. As for whether or not it's opinion based (one of the people voted to close based on that), our site relies more on opinions/experiences than some, which is one reason why it succeeds. Unless this gets piled up with low quality answers, that might not be an issue here.
    – Sue Saddest Farewell TGO GL
    Commented Nov 12, 2016 at 23:02
  • Usually review that start with "called the company to make sure i knew how to use the product" and end with "will not use anymore" should be taken with a bit of salt. It obviously is an inexperienced person and all sort of mistakes could have been done along the way. Trust reviews from people that have enough experience to say if something is good or bad.
    – Erik vanDoren
    Commented Nov 16, 2016 at 18:03
  • And this stuff has a limited shelf life and requires specific storage conditions, do you know how it was shipped?
    – Erik vanDoren
    Commented Nov 16, 2016 at 18:10

1 Answer 1

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There are several factors that can inhibit the gelcoat from curing. There is not enough information in the review to identify the specific cause. The only specific cause mentioned is stirring, if we assume the catalyst (hardener) and gelcoat where well mixed, here are some other potential causes (most common, not exhaustive).

  • Old Product Both the epoxy gelcoat and catalyst have a shelf life, if they are to old, they will never fully cure. Prevention buy new product from a reputable volume dealer.

  • Not Mixing Properly To much coloring agent or to little of the catalyst can inhibit curing. A new wood stir stick can absorb the catalyst. Prevention Pre-stir the gelcoat, then add the catalyst and stir well, scrape the sides of the container while mixing. If spraying do not over thin.

  • Temperature Gelcoat should be applied when temps are about 70F-80F or 21C-26C. The boat, product and air all need to be warm/cool. Prevention work in a heated/cooled area. For big boats outside, warm/cool the outside AND inside of the boat. Follow the manufactures recommendations to adjust catalyst for temperatures

  • Moisture (humidity) High Humidity inhibits curing, over 60-80% is problematic. Prevention work in a environmentally controlled area, Air conditioning also removes heat and moisture. In a heated area use a dehumidifier.

  • Clean Surface Follow manufactures directions for cleaning before and during application. Some products release amine during curing resulting in amine blush Prevention If directed; wash with soap, water and scrub pad between applications. Amine is water soluble sanding and/or acetone wash without soap and water washing only spread the amine. Warm dry environments reduce blush from epoxy products.

Solution If your gelcoat has not set in 4 to 8 hours there are two options.

  1. Limit oxygen exposure. The easiest way for the backyard/garage home repair person is to cover the area with wax paper. If it is warm and dry and another 4 to 8 hours has not caused it to set, see step 2. Example from a couple months later Wax paper on new application

  2. Scrape off the uncured gelcoat, sand and prep. Figure out what the problem was and reapply after addressing the problem.

A couple of references that address the many potential problem issues. Both have ads but serve to identify areas of concern

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  • The answer above is result of recent, extensive online research. It summarizes the common threads of multiple sources. Most of which are not included as references to the answer.
    – James Jenkins
    Commented Nov 12, 2016 at 11:59
  • The reasons for not curing can be a lot, sometimes its a mix of reasons unfortunately unless you have a step by step description on what the person has done its hard to find the cause unless there was some big mistake. With forums and workshops sometimes the cause is found only seeing the person doing the job. sometimes a sweaty hand on the surface ruins a coating so go figure
    – Erik vanDoren
    Commented Nov 16, 2016 at 17:58

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