Hinesy wrote:Time to patch up a few mistakes on our blue hull this Spring. I ordered a pint of blue liquid Gelcoat from the factory...how much MEKP should be mixed with the gelcoat?
Hello, Hinesy. The article Beene posted is a good one, but it misses a few points, one of which is the question you've asked, above. First, it's important to note the ambient temperature... as a thermoset polyester resin, gel coat is going to be highly dependent on the surface and ambient temperatures - the hotter it is, the quicker it's going to kick off. So, letting that be your guide, at room temperature (70-72°F) generally you'll add around 12 drops of hardener (MEKP) to 1 ounce of gel coat (or 6 per ½ ounce). Almost all gel coat requires 2% hardener per volume, and that's about the correct amount.
Weigh the gel coat carefully on a scale (a kitchen food scale usually works well for this). If your temperature is more than that, back off just a little bit on the hardener, or if it's a bit chilly, add a bit more - the maximum you should add is around 18 drops per 1 ounce if it's very chilly (50% more than the standard useage). [Oh, and remember to weigh the empty cup first, and subtract that from your gross weight... at the small portions your using, this can make a significant difference.]
Never apply gel coat when the temperature is less than 60°F. And above 85°F you'll find it difficult to use any thermoset resin, as it will kick off faster than you can apply it properly!
If you will be working in cool weather, it is useful to warm the surface with a heat lamp, but I find it's best to warm the surface first for a while to get it to a stable temperature, then begin working - remember, you're aiming at a surface temperature of 72°F. Other folks like to apply the resin and then heat it, but IMHO that often leads to uneven cure rates in the layers.
One important tip - stir the gel coat thoroughly (in the can) before you pour any in your cup. Like paint, you need to stir it before you use it! And, obviously, don't use a stirrer that's been
anywhere near MEKP! It helps to use a Sharpie marker to label your stirrers if you're doing multiple batches.
I find it is easiest to add the MEKP drops to a separate cup, and then dumping that into the gel when you're ready to start. This prevents accidentally squeezing too much into the gel and also let's you time your start a bit better. When you use this method, add one more drop of MEKP, as about that much will remain in the cup.
Take about a minute or so to stir the product and make sure the MEKP is fully mixed into the gel... remember to scrape the sides & bottom of the cup thoroughly to get all the gel into contact with the MEKP. You won't be able to see the mixed product turn color like with an epoxy, so you'll have to be extra dilligent at this stage. You don't want to be at the end of the cup and be applying gel that didn't get any hardener in it! Very messy.
BTW, plastic yogurt cups work really nicely in terms of size and utility in working with gel coat, and popsicle sticks (available at any craft store) work nicely for mixing. YMMV.
Hinesy wrote:About how long does it take for the mixture to begin to harden up in order to reach the proper consistency to apply?
Again, it depends on the temperature, but generally if you've mixed things properly you should see it start to harden in around 20 minutes - and you start counting from the second the MEKP hits the gel, not once you've stirred it all in. This, of course, includes the time you're working the product, so work quickly and don't try to mix a big batch and do all your scratches at once - you'll run out of time and waste your gel.
If you'll be doing several batches, carefully time each one... this will allow you to better measure the amount of MEKP required for the next one... if you're going long, add more, and vice versa.
Also, as soon as you feel that gel start getting clumpy or thick in your cup, stop applying it and/or working the surface. Don't be tempted to try and apply that last bit really quickly... you'll just end up with a clumpy mess on your boat that never quite stuck to the surface properly. Better to waste some gel than to create a mess you'll have to sort out!
[On edit, one note: Be careful how you hold the cup... if you grip it in your fist you will allow a considerable amount of your body heat to transfer to the gel in the cup, thereby accelerating the cure rate of the stuff in the cup. Only hold the cup when you need to, and never hold it where the gel is (low on the cup)... always hold it above the gel.]
Hinesy wrote:And how long to wait before coats if more than one is needed to properly fill (slightly overfill) the scratches?
I usually wait until it starts to get a bit gelled, where you can still easily dent it with a fingernail.
Important note:
One part of that article is somewhat buried, but it is
vitally important. Gel coat is not made to harden in the presence of air - after all, it's usually applied to a mold and then fiberglass and resin are applied on top of it, sealing it in. So, when you've applied that top coat, immediately cover it with either wax paper or saran wrap to block out the air,
while the gel is still wet. This way you'll be assured of getting a full cure, and not ending up with an eternally sticky patch of gel that catches every bit of dirt that hits it! If you've ever seen a 'dirty' repair on a boat - one that never seems to clean up properly - that's the reason.
Safety note:
MEKP is a strong eye irritant. Even the fumes can make your eyes burn, much like with onions. Use plenty of ventilation and always use eye protection when working with it.
Further, I can't count the times that I've seen guys puncture one of those tubes improperly and they end up with it not quite fully punctured (and they squeeze it really hard to get it to drip) or not punctured straight in, and it ends up squirting out at some unexpected angle or splashing up at them. A dose of MEKP in the eyes will definitely ruin your day, so please use appropriate protection.
Good luck!
Kevin
(PS - sorry this answer was a bit delayed - I was off camping all weekend with a bunch of Boy Scouts!)
