Thread: Fibreglassing (carbon fibre) - cut holes before or after?

A carbide router bit will hold up just fine, but you should probably use a proper bit for composites, not a normal spiral bit
Burr style cutter for fibreglass | CNC Router Bits

You probably want to avoid having anything sitting on the finished surface while routing as I'd guess that any dust/chips that get pushed along by the router base will scratch the heck out of the surface, or just factor in sanding back the surface and re-applying a finish coat afterwards.

Glad to hear that - I was very much hoping to avoid precutting.

I’ll either do the routing between finishing coats, or I’ll stick some contact on it to avoid scratching.

Ouch - those bits are quite pricey ( not surprising given solid carbide)


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I'm a bit confused by your description.
Are you intending to wrap the heat sinks in carbon fiber -- doing so would seem to prevent them acting as heat sinks.

If you are after a black finish, an alternative to carbon fiber is to use standard fiberglass mesh and tint the epoxy resin black.

ahh no, that would be somewhat foolish wouldn't it. The ply frame is glassed and the heatsinks are sitting in the space in the middle.


The heatsinks are black anodised aluminium so are a little hard to distinguish from the carbon fibre in my render.


Here are some other views which may (or may not) make it easier to see the end product. In one of the images, I have removed the heatsinks so you can see what is actually being glassed (the white strips are just un-textured elements - they are some risers to keep the heat sinks at the correct height).

It's not shown in the wireframes but there is an end cap at each end covering over the square channels and end grain - hence the closed carbon fibre ends on the finished product. The end caps are separate forms that will be glassed and attached afterward so they can also be removed.

I'm a little unsure how to go about removing the carbon fibre from the lighting holes (which are already cut in the form) afterward... I may need to drill a decent size hole through and re-cut with the bevel bit I originally used on the router.

Aquarium Fixture 7.jpgAquarium Fixture 6.jpgAquarium Fixture 5.jpgAquarium Fixture 4.jpg

In the recent past I built a stitch and glue kayak.
based on that experience, I suggest you cut the recesses after the first wet-out coat of epoxy has cured but before you apply any fill coats.

With 6 oz (per sq yd) fiberglass fabric I found the freshly cured fiberglass cut easily with standard wood working tools -- files, rasps, cabinet scrapers, etc.

That's great to know Ian, cheers. I was planning to pretty do exactly that. So glass it, then sand back lightly and while it's still scuffed, do my cutting, then do fill coats. That way, there is no undue scratching from the cutting tools that wont be covered over. Kayak building has been on my mind while making the form for this as I'm strip planking the curved/round sides. I have a set of plans for a Guillemot strip plank kayak in my cupboard that need to come out some time in the near future. Very much looking forward to that project. I got plans for the petrel sea kayak - it looks very nice.

I built a Pygmy Boats "Ronan" Ronan Kayak Kit: Playful and Lightweight

Looks nice, design sounds like it’s designed for similar purpose to the petrel.
https://www.guillemot-kayaks.com/guillemot/node/89/pics

I prefer the strip plank design as you get nice curved surfaces, but it’s more work when building.

Guillemot Kayaks has a great YouTube channel with a bucket load of instructional videos on all kinds of building techniques.


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A few friends of mine worked with carbon fibre and Kevlar for a living, Stanley knife was always used to cut excess after first coat, scissors to cut the cloth to size prior to coating. Yes you will go through a few blades but it works.

Thanks Cal,

Originally Posted by Cal

Stanley knife was always used to cut excess after first coat

Do you know if this was done before it had fully cured?I can kinda imagine doing this while it's touch dry but still a little soft/flexible.I think this will be good if I can cut out the light holes like this after the first coat.

Originally Posted by woodman-79

Thanks Cal,Do you know if this was done before it had fully cured?I can kinda imagine doing this while it's touch dry but still a little soft/flexible.I think this will be good if I can cut out the light holes like this after the first coat.

my experience is that the glass was quite easy to cut for the first 48 hours or so.
The epoxy I was using cured sufficiently to allow further work or subsequent coatings after about 12 hours -- so a coat applied in the evening was dry and workable the next morning. Full cure took 3 to 4 days, but you needed to get subsequent topping coats applied within 72 hours of the first fill coat. Typically, I'd wet the fabric out on day 1, and apply the topping coats on days 2 and 3.

So if i were attempting what you are, I'd fully wet out the fabric to make it stick to the substrate. Leave it till the next day, cut out the openings, clean up any residue, (but not worry about any "scratches" as attempting to sand them out will damage the fabric), and then apply the first of the filling coats.
On day 3 I'd be looking at starting on subsequent fill coats.

Now the epoxy you are using (and your ambient temperature) is almost certainly different to what I am used to so my timings might need adjusting for your situation. I had one coat that because of low temperatures took almost 24 hours to get to tack dry. I turned a heater on for subsequent epoxy applications.

Cheers Ian,

I have a question though.
There are two types of cutouts I need to do;
1. I need to cut the fibreglass out of the existing holes where the LED's stick through the base - this I think will work well using a razor/stanley knife and following your suggestion and Cal's advise.
2. I need to rout out some recesses/access panels through the fibreglass as well as the ply substrate.

Do you think cutout #2 would also be done at the time you suggested, or would you wait for all coat's to be done then cut that out.
I just down want the router to pull the fabric at the edges and make a mess if its not fully hardened.

..and apologies for the long line of text.. for some reason returns are not working on this forum for me anymore. It was properly formatted when I wrote it.
EDIT - fixed with some HTML....which the posting permissions says is turned off....

Yes cut while still flexible usually.