Hey folks,

I had a first attempt at carbon fibre skinning at the weekend and everything seems to have gone really smoothly apart from one issue.

We were skinning plastic parts, with a fairly rough surface to begin with, but still did a bit of a key with 120 grit sandpaper to be safe. I'm not sure exactly what type of plastic the parts were made from. At first, the basecoat went on really well but within a few seconds it started to split a bit, as if it was being repelled from the surface. For the majority of the part it didn't cause any problems but had prevented the carbon fibre from adhering properly to the part completely. The majority of the part is absolutely fine but there are little segments that didn't stick down as well. The fibre is being wrapped around the surface entirely, so I'm not worried about it delaminating and detaching completely, and the epoxy layers on top have applied really well. The issue came where the joins of the cloth didn't adhere well so it's not quite as neat as it could be. 

Unfortunately , we don't have much scope for using different types of parts - this is just a bit of fun, rather than for commercial sales or anything, but getting a perfectly finished part would be very satisfying. 

Is there any sort of primer or pre basecoat that we might be able to use that the basecoat could then adhere to?

Thanks!

It's simply a lack of adhesion. Plastic parts tend to be very low surface energy, which means they are difficult to bond to and you need to be extremely thorough with your prep work. It doesn't matter if the surface is textured, you need to do a thorough key all over with 60 or 80 grit. 120 grit is fine for most applications, but for plastic I would go rougher. Even doing an all over key with 60 grit and then deeply scoring the surface with a blade is a good idea.

You also want to make sure you degrease the surface extremely well before AND after you sand. Be careful using acetone for this, as if acetone effects the plastic then it will actually remove your key. Wax and grease remover is a more gentle solution.

Yes, this can happen if the plastic you're attempting to skin is something really difficult to bond to, the surface of these plastics can behave like it's trying to repel the resin. Examples of this would be the polypropylene plastic mudguards on motocross bikes; they are completely repellent to resin, in fact polypropylene is what we often use as a release barrier because epoxy simply will not stick to it. In this situation then all bets are off and you just have to do what you can to get the basecoat to take to it. If the situation was more critical and the epoxy skin *needed* to bond to the plastic properly then you'd probably have to give in and accept that you're not going to get an adequate bond but as you've already explained that you're wrapping the carbon round (i.e. encasing the part) and that the quality of the bond is not a concern in this instance then I think you're OK to just do what you can to get the resin to hold temporarily.

Your best options would be, as Hanaldo says, some extreme keying with abrasive. I would probably suggest leaving it at a VERY coarse grit (maybe 80) and make sure that it's scuffed to within an inch of its life. When you scrub low surface energy plastics like polypropylene and polyethylene they tend to not sand away but instead the surface gets a bit 'woolly' and this can work in your favour because it makes a very complex surface for the resin to flow into and around, helping it to get some sort of mechanical bond onto the surface.

I hope this helps

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Thanks for your replies. 

There is a very good chance I didn't do enough prep work on the surface of the parts so when I attempt it next time I'll be sure to go a bit overboard with my surface prep. 

For the vast majority of it, it was no problem not having full coverage. Of course, the bit where it did matter happened to be where I was attempting to have the fabric join together. This meant that it didn't sit completely flat so when I sanded it down it did leave a bit of a gap, which then meant the pale plastic is a bit visible.

It's all good practice though and I'm really enjoying working with it. It's not quite as bad as I thought it might be!