Motorsport Composite Fundamentals: Applying Gelcoat

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Applying Gelcoat

05.01

00:00	With our release agents applied and fully cured, the next step is applying our gelcoat.
00:05	Specifically tooling gelcoat for our mould, which will provide a durable, hard-wearing surface that we can also sand and polish smooth and glossy for a good finish.
00:15	Depending on the size of the parts we're working with, we pour a suitable amount of gelcoat into a mixing container.
00:21	We want the gelcoat to be about half a millimetre thick.
00:24	So, this means that we'll use about half a litre for every metre square of area.
00:29	Our fenders probably have half a metre square of surface area, if that.
00:33	So, we started with 400 millilitres of gelcoat in our container.
00:38	And remember, we can always mix more if needed, and we don't want to have too much excess that's going to be wasted.
00:44	The gelcoat needs to be catalyzed at 2% with MEKP.
00:48	So, with our MEKP in our catalyst dispenser, we need to measure out 8 millilitres in the flute.
00:54	Keep in mind the considerations we discussed around working with MEKP in a dispenser from our practical pattern making modules.
01:01	Check back if you need a refresher.
01:03	Aim the bottle away from your eyes when filling the flute, and consider any retention of the catalyst in the flute that may need to be accounted for.
01:11	Once we've added 8 millilitres of MEKP to our 400 millilitres of tooling gelcoat, we need to mix thoroughly.
01:18	A clean wooden mixing stick or metal ruler makes this an easy job.
01:22	We should be mixing for about 2 minutes, constantly scraping down the sides of the container to ensure there's no uncatalyzed pockets of gelcoat that won't cure.
01:31	Once it's fully mixed, the pot life is only around 20 minutes, so we need to get to work.
01:37	It's best to complete one fender before the other because we can always stop, mix up another batch of gelcoat and start fresh.
01:44	Now, we're ready to apply it to the surface of our pattern over the dry PVA parting film.
01:49	Again, this can be done in a variety of ways, and while a professional workshop would typically choose to spray it on for the most efficiency and ability to create an even thickness, for this example we've chosen to show the most accessible approach of brushing it on.
02:04	For this we'll use cheap disposable chip brushes, but those that won't leave any bristles behind in our layout.
02:10	The aim is to apply an even coating in full coverage, again at a thickness of about half a millimetre or two hundredths of an inch.
02:19	Like we discussed when applying our primer, this can be checked with a film thickness gauge.
02:24	If we place this gauge on a flat surface, the number of teeth in contact with the gelcoat will indicate the thickness.
02:30	It's important the gelcoat is thick enough to prevent print through, where we get transfer of the reinforcement texture to the mould surface, and also provide a durable surface with enough material to sand and polish smooth.
02:42	However, if the gelcoat is too thick or simply an uneven thickness, these areas won't cure properly and we could end up with what's called tripe, where the gelcoat on the surface of the part wrinkles like alligator skin.
02:54	Again, it's a simple matter of brushing the gelcoat onto the surface, using a mixture of different brush sizes depending on the details of the part, and being careful around the edges and internal corners to avoid thin spots, feet puddles or runs.
03:08	Don't forget to include the flanges either.
03:10	After application, the gelcoat needs to be left for 4-8 hours or more, ideally overnight for cure.
03:17	Remember the gelcoat won't fully cure to a hard backing surface, it'll still be slightly tacky to touch, but won't transfer to our hand.
03:26	If it does transfer to our hand, or we can very easily push dents into the back of the gelcoat, then it's safe to say it's not cured and we need to give it some more time.
03:35	An additional coat of tooling gelcoat can be applied after the initial layer has cured, also at 0.5mm thick, to ensure enough surface material is available to sand and polish flat and smooth, and also ensure durability.
03:49	In reality, this mould will likely only be used a handful of times, so we've chosen to forgo the extra coat in the interest of saving time and gelcoat while keeping things simple.
03:59	We need to make sure we dispose of any excess gelcoat as per our discussions in the fire safety module earlier in the course.
04:06	Any large amounts need to be dispersed, and make sure all gelcoats and resins are fully cured in a low-risk environment before going in the trash.
04:14	We're finally ready for the lamination stage, but before we move on with that in the next module, let's summarise what we've covered so far.
04:21	After our release agents are fully cured, the next step is tooling gelcoat.
04:25	If we're aiming for 0.5mm thickness, we need about 500mm of gelcoat for every metre square of surface area.
04:33	This should be catalysed at 1.5-2% with MEKP, but it's important to follow the supplier's instructions here.
04:39	When thoroughly mixed, it can be sprayed or brushed onto the surfaces, keeping in mind the limited pot life and working for an even film thickness, not too thin or too thick.
04:49	After application, the gelcoat will take about 4 -8 hours to cure, however it's best to err on the side of caution and leave it overnight if possible.