Motorsport Composite Fundamentals: Topcoat & Other Coatings
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Topcoat & Other Coatings
05.56
| 00:00 | In the previous module we discussed protective coatings in the form of gel coats, that are applied to the moulds and bond to the surface of the parts during construction. |
| 00:09 | The coatings most of us are more familiar with will be applied to the surfaces of the parts after construction as somewhat of a finishing process. |
| 00:18 | Top coat, also referred to as flow coat, is essentially a gel coat that's applied in this manner, and it should be noted here that many people still refer to these as gel coat. |
| 00:29 | As we mentioned in the previous module, as the gel coat dries in the mould, it will often have a tacky finish. |
| 00:36 | The outer surface is of course sealed from air against the mould, and the inner tacky surface will be covered in our layout. |
| 00:43 | As the part cures, everything bonds together, and as we demould the part, we have a nice hard outer surface. |
| 00:50 | If we painted gel coat on the outside of a finished part, it will have this tacky nature, although this will eventually go away, in the meantime it'll likely attract dust and dirt. |
| 01:01 | Top coat is very similar in chemistry to gel coat, but is formulated with a small amount of mineral wax to prevent this. |
| 01:07 | So, as it cures, this wax rises to the surface, creating a barrier to the air, so it hardens to be tack free. |
| 01:14 | We can also manually add mineral wax and styrene solution additive to our gel coats to create a top coat or a flow coat. |
| 01:23 | There are a few things to consider here, first is the bond between the coating and the surface of the part. |
| 01:29 | Since they're not curing together, they won't chemically bond, so they're limited to a mechanical bond. |
| 01:35 | A rough surface finish will help increase this mechanical bond, so keying the surface with sandpaper is recommended before applying any top coats. |
| 01:43 | The other factor is that the coating isn't completely self-leveling. |
| 01:47 | The top coat is usually brushed on, although it can be sprayed on, and the brush strokes will flatten out somewhat, but not completely. |
| 01:55 | So, if we want the surface to be smooth, it takes a fair amount of sanding and polishing that's not required as much with gel coat. |
| 02:03 | For these reasons, and of course that it can be applied in this manner as opposed to gel coat, we often see top coat used on the back surface or open face of our composite parts, in other words, not the mold surface. |
| 02:16 | Whether the surface was cured open to atmosphere or in contact with peel ply, it'll most likely be significantly rougher than the mold surface, which conveniently helps provide a good mechanical bond as we discussed before. |
| 02:29 | The inside of fiberglass over fenders is a good example of this. |
| 02:33 | The top coat helps seal the surface from all the road debris and moisture it'll be exposed to, especially if it's been sanded at all and any fibers are exposed. |
| 02:43 | Let's move on to more conventional forms of coatings like paints and clear coats. |
| 02:47 | In most cases we'll opt to use a 2k paint or clear coat. |
| 02:52 | Basically, this means the coating has two parts, the paint and the hardener. |
| 02:56 | These generally provide a harder wearing and more UV stable surface than 1k coatings, which is more suitable for automotive applications. |
| 03:06 | The downside is that they're generally more expensive and the application process is naturally a bit more involved. |
| 03:13 | We'll start with clear coats and these are generally used for two main purposes, UV protection and to give a very high gloss finish for acidic carbon parts. |
| 03:22 | Clear coats will also help fill small pinholes and imperfections, however the clear coat usually won't take well to any bare fibers, so these need to be covered in resin first. |
| 03:33 | The painting process will vary depending on the product we're using and we should always follow the supplier's recommendations here. |
| 03:41 | That said, the process usually looks something like this. |
| 03:44 | First we should clean the parts with isopropyl alcohol to remove any residual release agents and then sand with anything between 240 to 800 grit. |
| 03:53 | The final preparation step is to clean the part again with soapy water and a final degrease, usually with isopropyl alcohol or wax and grease remover. |
| 04:02 | Next, after activating the coating by mixing the two parts, however that may be, we can apply two to four even coats of the clear coat, allowing it to dry between each coat. |
| 04:13 | Following this, it's best to wet sand with something like 1200 to 2000 grit and polish to bring up a deep and high gloss finish. |
| 04:21 | Finally, let's talk about painting. |
| 04:23 | There are a few schools of thought here. |
| 04:25 | Some of you might wonder why you'd ever want to paint over your carbon parts. |
| 04:29 | If you did, how would people know that they're carbon fiber? Others will want to be hiding that gaudy weave, or if you're like me, then it really depends on the part in the car that it's going on. |
| 04:39 | Regardless, painting composite parts is much like painting any other part of the car. |
| 04:44 | Remembering in the end, these are basically plastic parts anyway, like most bumpers on modern cars. |
| 04:50 | So, we'll use the same primers and enamel paints. |
| 04:54 | Again, it's important to key the surface first with 240 to 800 grit sandpaper to give the paint or primer something to adhere to. |
| 05:03 | Clearly this isn't a how to paint your car course and I'm not a painter, so we'll leave it there and wrap up this module with a summary. |
| 05:10 | Top coat, also called flow coat, is essentially a gel coat designed to harden as it dries. |
| 05:17 | And this can be painted onto pre-cured parts. |
| 05:20 | This is more commonly used on the backside of fiberglass parts as it bonds better to a rougher surface and usually won't give a high quality smooth surface anyway. |
| 05:30 | Other coatings like regular car paints and clear coats for acidic carbon parts are also commonly used in composites. |
| 05:37 | We tend towards 2k paints for these applications due to their harder and higher quality surface finish. |
| 05:44 | Just be sure to prepare the parts with cleaning, sanding and more cleaning to ensure good results. |
| 05:49 | And polish the surface afterwards for a professional finish. |
