Motorsport Composite Fundamentals: Finishing Composites Parts

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Finishing Composites Parts

08.02

00:00	The surface finishing work on composite parts is going to depend on various factors.
00:04	One is the surface finish of the parts out of the mould, for example how rough it is or if there's any surface voids present.
00:11	Another factor would be the materials used in the construction, for example if a gel coat was used, which can be polished up, or maybe clear coat needs to be applied if we want a glossy finish that shows off the woven reinforcement.
00:23	Other parts might be painted, covering up the reinforcement.
00:26	And some parts, often dry carbon, might be left uncoated to keep the weight as low as possible.
00:32	While the fiberglass CRX fenders we've been working on throughout this section of the course will be painted at some stage, we'll shift our focus to a carbon fiber part where the surface finishing is a bit more rewarding.
00:42	Our fiberglass fenders also didn't have any surface voids, so let's look at a part that does so we can demonstrate how to fix them.
00:49	These are some carbon fiber versions of the same fenders, which have been made via the same hand lamination techniques we've been looking at throughout the course.
00:57	The construction process has been documented in the worked example library, so I'd recommend checking that out after you've finished with the main body of the course.
01:04	We used a polyester laminating resin, and we went without a gel coat in this case, to keep things as simple, lightweight and cheap as possible, but we'll clear coat the part for the sake of showing a different approach.
01:16	What's important to understand here is that it's very difficult to hand laminate woven carbon fabric into molds with tight curves, like on our fenders, as the reinforcement doesn't conform easily.
01:27	So,, without a vacuum bag to hold the reinforcement in place, the woven reinforcement tends to peel itself back up.
01:34	The result, as to be expected, was some surface voids, and as we'd also expect from a hand laminated, open-cure part with no gel coat, we also have a good deal of surface pinholes.
01:44	The first step to repairing the surface voids is to sand and key the surface of the part around the voids with 400 grit sandpaper.
01:51	After sanding, we wipe the surface of the part down to remove any of the sanding dust, and then apply flash or release tape just under the edge of the void to create a dam, which is going to hold the resin in place while it cures.
02:03	We can then use our laminating resin, or a coating resin if we intend to use that to coat the part, which we'll come back to shortly.
02:09	In this case, we'll be using the same laminating resin that we used to make the part.
02:13	This is a waxed resin, and is catalyzed between 2-3% for this job.
02:18	It's difficult to accurately catalyze such a small volume, but it's not a problem for such a small, quick repair job if it cures fast.
02:25	Using a popsicle stick to mix the resin, we then tease it into place, ensuring the void is completely filled.
02:31	Any excess will have to sand back once it's cured, so it pays not to go crazy and create more work for ourselves.
02:36	Once cured, after around 6 hours for this particular resin, we can remove the flashing tape, and then sand the repairs back to match the shape of the part, starting with the same 400 grit that we used earlier.
02:48	To prepare for our specific clear coat, we then sand up to 1200 grit, using the DA for efficiency, and an interface pad.
02:55	This is a soft foam pad, which helps the sandpaper conform to the surface easily, and apply even pressure, without risking changing the shape of the surface.
03:04	Keep in mind that this preparation will vary depending on the coating product you intend to use, but if you're ever unsure, the supplier should state this on the TDS or technical data sheet.
03:13	Following this, we need to make sure the parts are perfectly clean before they're coated.
03:17	Either a thorough wipe with a wax and grease remover, or a wash with soapy water and a blow dry.
03:23	It's important to remove all the sanding dust from the pinholes, as this can react with the coating when it's sprayed on.
03:29	The next step is coating the part, and there's a few options if we want to keep the woven reinforcement visible.
03:34	We can use an automotive clear coat, or a clear coating resin.
03:38	Either will provide a durable finish, UV protection, and some material to polish.
03:42	We'll use a general automotive spray on clear coat called Diamond Chrysler Clear 7900, which is a 2K or 2 part coating, ideal for the most hard wearing finish.
03:53	We'll mix this as per the supplier's instructions at 4 to 1, before transferring it into our spray gun pot.
03:58	So, for every 100ml of part A, we add 25ml of part B.
04:03	Again, we can follow the supplier's instructions for the spray application process, applying 2 coats, with about 10 minutes flash time between the coats.
04:11	Each coat should feel relatively light, but still look wet.
04:14	As always with painting, this is something you get a feel for with experience.
04:18	We're looking for more than a dusting, but if we overdo it and get pooling or a run, we need to stop, allow the coating to cure, sand back the run, clean off the dust, and reapply at a lighter rate.
04:30	After 2 coats, we then allow the parts to dry for at least 4 hours.
04:34	The surface finish at this stage is relatively good, but there's always room for improvement.
04:38	Although these fenders will likely be wrapped or painted at some point in the future, it'll be nice to get them looking as good as possible for the time being.
04:45	So, we'll start by sanding with 800 grit, and then working up to 2000 grit wet.
04:50	And then we'll move to a cutting compound with our polishing machine and a foam pad, wiping off the leftover with a microfiber cloth.
04:56	The alternative to spraying a clear coat is to use a coating resin, which will typically be epoxy based and can be mixed with a hardener and then brushed or sprayed on.
05:04	Brushing has the downside of leaving stroke marks, and although they self-level to some degree, they will require a bit more flatting with sandpaper to get a smooth finish.
05:13	If we're using multiple layers of a coating resin, it's important that we let the first layer gel to a slightly tacky finish, but not fully cure.
05:21	This will allow the second layer to create a better bond with the first.
05:24	While epoxy coatings can generally be applied to epoxy, polyester and vinyl ester composites, flow coats, which are typically polyester based, can only be used on polyester and vinyl ester composites and shouldn't be used on epoxy composites as we're more likely to run into adhesion issues.
05:40	Recalling our gel coat discussions from the earlier materials section of the course, flow coat is essentially a gel coat, but it's applied to the surface of a cured part, with the main difference being that it contains wax to ensure a hard surface cure.
05:53	These are catalyzed at 2% with MEKP and brushed or sprayed onto the surface.
05:58	In any case, when the coating has been applied and fully cured, and this goes for gel coats as well, it can then be wet sanded up to at least 1 ,200 grit, ideally about 2,000 grit, and then cut and polished for a high gloss finish.
06:10	Generally, speaking, a polyester surface will be the softest, vinyl ester will be slightly harder, and epoxy will have the hardest surface finish.
06:17	This affects the approach taken to working on the surface, where we use more aggressive brits as well as firmer polishing pads and pressure on a harder surface, but still work up to a fine abrasive to achieve the high gloss finish.
06:30	That said, some coatings will be applied with the intention of a matte or satin finish, in which case they obviously won't be cut and polished.
06:37	Just like we should wax our car paint after polishing to protect it, this is recommended specifically for carbon fibre body panels that will be exposed to the elements, and the process is much like that of applying mould release wax.
06:49	Wipe a coat of wax on, allow it to haze up, and then buff it off with a clean microfibre cloth.
06:55	This hasn't been an exhaustive list of every single finishing process we can apply to the surface of our composite parts, but more of the typical approach when we're trying to show off the woven reinforcement, so let's summarise this module.
07:07	If our hand laminated parts have any surface voids, the first step is going to be to fill them.
07:12	We start by keying the surface with 400 grit and then cleaning it thoroughly, before using release tape to make a dam and then teasing some catalysed or activated resin into place.
07:22	When the repair has cured, it can be sanded back to match the shape of the part.
07:26	In the absence of a gel coat, we'll often add a clear coat to our parts for protection, and to provide a surface that we can polish to a high gloss finish.
07:33	Again, the surface needs to be keyed with sandpaper and thoroughly cleaned, and then the clear coat can be brushed or sprayed onto the surface as per the supplier's instructions.
07:43	The coating we use needs to be compatible with the resin system used in the construction of the part, but no matter if it's an automotive clear coat, coating resin, flow coat, or our parts have gel coat on them, they can then be dry and wet sanded up to at least 1200 grit before being polished.