Guide to cutting, trimming, shaping and finishing carbon fibre parts

Achieving a high standard of trim can make the difference between a Carbon Fibre component looking totally professional or looking tatty. With proper care and only basic tools a perfect finish is surprisingly easy to achieve. In this guide we show you how.

To achieve a neat and accurate cut, first apply masking tape to the area you wish to trim, this will stop scratching and also make marking easier. Needless to say measure and measure again the cut you wish to make, the last thing you want is to trash an expensive piece of carbon because you rushed the marking, mark the line accurately with a pencil, give yourself approximately 1mm oversize this will mean any small fractures can be sanded off later.

Carbon composites are relatively easy to cut; they are only plastics after-all. There are a range of diamond and tungsten carbide cutting tools available specifically for composites, these are ideal for the process as they won't pull and chip at the trim-line, however with gentle use a FINE toothed hacksaw blade or fine files will do the job. When using any tool apply very little pressure and work with the high-finish side up. Take your time.

Now you have cut your component (slightly oversize) it’s time to neaten the cut by sanding it, if you have straight lines we strongly recommend the use of a Perma-Grit sanding block, again however, excellent results can still be achieved using good quality ‘wet and dry’ type sand papers on a block, we would recommend around 180 grit paper as a starting point. Whenever sanding a composite, if possible, sand down the line of the cut, this will give a flat and neat edge. In the case of complex cuts and holes use a fine file and work in steady strokes working away from the high finish side only.

Once you have the cut-line true and accurate, lightly work around the edge with a fine sand paper (around 400 grit) this will soften and smooth the edges.

To get a really top quality edge to the cut the secret is simple; apply a light automotive wax to the edge and buff away, this will also help to seal the edge from water ingress.

Alternatively, if the composite you have cut is not 100% carbon fibre (perhaps it is one layer of carbon fibre and then multiple layers of woven glass) then the use of a black wax crayon on the edges will help to make the composite look a consistent black all the way through. Edges finished with a black wax crayon can be polished amazingly well - truly a secret of the professionals!

If you are feeling brave you can apply cyanoacrylate (super glue) to the edge instead of wax but be warned; this can result in making the edge look terrible if not done very neatly. Apply it with a piece of cloth and try not to stick yourself to it! After the glue has cured it will need a light sanding and polish/wax.

The cut edge on a sandwich panel will expose the core material and composition of composite. The cut edge will look unsightly and will also be susceptible to water ingress and vulnerable to damage. The professional way to seal the edge of a cut sandwich panel is to route out about 3mm of the foam core and then fill this cavity with an epoxy filler. In the case of a carbon fibre sandwich panel the use of a black epoxy filler which can then be sanded to a smooth gloss black is a very professional way to finish a panel and looks fantastic.