Gel coat/Catalyst ratio for small batch

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Schuey

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Jul 7, 2021
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Nova Scotia
My gel coat calls for a 2% mix of catalyst. I mixed 125ml with 2.5ml. It's been a few hours and has cured enough to barely take a finger print.

125 ml was way more than I needed. It took around 20 drops of catalyst to get 2.5ml. I'm thinking it would be safe to reduce those measurements in half?

Anybody have a rule of thumb for mixing a small batch of air dry gel coat?

Is there a trick to pouring this out into a mixing tub without making a mess?

Thanks
 
Does your gelcoat have air dry added to it?

Gelcoat is usually pretty forgiving of a hot mix (within reason) so I'd be tempted to mix up a batch that's heavier on the catalyst and work fast. Obviously ignore this if you're working with a large area.

Is your work area heated?
 
I second @CPS ' questions / suggestions.

Re: waxed / air dry: Polyester resins and gelcoat only cure fully in an anaerobic situation. In kayak production unwaxed gelcoat is sprayed into the mold which is the barrier against contact with air, and then the next layers are built up on the inside. For repairs you need to create that air barrier on the last coat. Add air dry wax (the wax will rise to the surface, creating that barrier), or if it's a small enough area tape a polyethylene film over it (zip lock bag...)

While it might eventually cure as long as it doesn't freeze, gel coat doesn't like temperatures below 10*C. I work with the ratios in the chart found here: https://fibertek.ca/shop/products/fiberglass-materials/m-e-k-p/ and, for (very) small batches, use those graduated 1oz medicine cups, "spooning" out from the can with tongue depressor used as stir sticks.

Another point to consider: gelcoat and MEKP both have relatively short shelf lives, only buy as much as you are using in the next 6 to 12 months.

Edited to add: Once you think you have done a good job mixing it thoroughly, go back and make sure that you are scraping the wall of the mixing container and getting the gelcoat off the stir stick, and then mix some more... ;-)
 
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The gel coat is an air dry. Checked it this morning and it's super hard. Hopefully it sands and sticks OK. I roughed it up with 80g. Would you use a coarser paper?

The boat is in my basement. Probably around 60-65f.

I'll try halving the amounts and lean towards the hot side with the catalyst.

Thanks to everyone for the advice. This forum has been a real boost.

Happy Holidays!
 
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Glad to hear it set up.

Regarding coarseness of sandpaper: I usually use 80 for any significant removal of material, then 120 for smoothing out the gauges left by that, and finally 800. You can definitely you go for something finer, but I find once the 800 wears down a little it's a finer cut than a fresh piece of 1200. Finally I use a cut polish to shine things up.

In some instances I'll even use a scraper first, as I find it does a good job of levelling a surface.

I am definitely more inclined along the thought that a kayak is a tool, not a jewel, so am tolerant of a somewhat heavy handed approach.
 
Schuey said:
My gel coat calls for a 2% mix of catalyst. I mixed 125ml with 2.5ml. It's been a few hours and has cured enough to barely take a finger print.

125 ml was way more than I needed. It took around 20 drops of catalyst to get 2.5ml. I'm thinking it would be safe to reduce those measurements in half?

Anybody have a rule of thumb for mixing a small batch of air dry gel coat?

Is there a trick to pouring this out into a mixing tub without making a mess?

Thanks
Click to expand...
Filling the chips and inevitable scrapes is an annual event with my fibreglas kayak. I bought a 500ml can of WAXED gelcoat five years ago. It tends to seperate a bit over time but is usable with a thorough stirring. Use the small clear plastic medical dispenser cups to measure out 10 - 15 ml batches. Each 10 drops of catalyst is equal to 1 cc in your calculation of 2%. After a thorough scratching the edges of the chip with 60 grit sandpaper and a wipe with acetone the chip is ready to be filled. I cut a spreader/squeegee from a plastic yogurt or sour cream container using tin snips. With a matching profile to the area being filled, I spread the gelcoat slightly proud from the chipped area. Once set, I use a small block of wood and 120 grit sandpaper to level the new gelcoat. This is followed by 320, 600, 1,000 and 1500 grit WET sanding with a small amount of dish soap and warm water, paying attention to rinsing between grits. A final polish with 2 coats of Mequiars wax leaves a very smooth finish that is hard to see.
 
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