Somewhere between woodwork and framebuilding: bamboo bike #2

FINALLY FINISHING THE CARBON

After many weeks of doing the carbon fibre, I was getting pretty bored. I feel like this reader's wife and I are celebrating our diamond anniversary. It was nice to finally get it done. After all the carbon fabric is on, it's time to put on (at least) two thick coats of epoxy to create a surface that you can sand down to be fairly flat. As I said before you'll never get the finish you might see on a moulded CF bike, but you can get a pretty nice result. At this stage you'll start to see how all the little defects and imperfections don't look so obvious now it's all shiny, and how the weave of the carbon comes out. However, the surface will still be a bit rough:

2 thick coats of epoxy later... by comatus1649, on Flickr

You can also see little bubbles left in the resin. These can be greatly reduced by a combination of heating the two resin parts before mixing, stirring very gently (definitely transfer it into a second pot if you're being very gentle as you'll otherwise have a bit of unmixed resin at the bottom), hairdrying the resin in the pot (this shortens pot life a bit - a roller tray is best to stop it curing very quickly) and then again when it's on the bike. The foam brushes also tend to froth up the resin so you can either change to normal brushes, or as I did, just go very gently. Beware of cheapo paintbrushes which dispense bristles everywhere.

A note on masking techniques - wrap loads of tape around the end of the lugs. This means that you can take off one wrap after each coat, and not have a massive drippy buildup when you're finished. It's probably easier to take off the tape when it's stiff but not cured, but I always put epoxy on before I went to bed so I always had to do it when it was dry - bit more tricky.

The seams between bits of fabric now look much subtler, but it looks pretty homemade:

Built-up epoxy by comatus1649, on Flickr

So it's now time for more sanding.

I hate sanding.

You're taking off a lot of epoxy so begin with the rough stuff - I began with 80 grit aluminium oxide paper. Reading the Carbonmods skinning guide it tells you to use 240 wetndry, which is all very well if you're taking out little lumps and bumps on an otherwise flat surface, but you need to build up a lot more epoxy on these complicated joints, so you'll need to take it off more roughly. This is also your last chance to knock off the high points caused by loose fibres, rough joints and other mistakes. I started with 80, then 150, then 400 wetndry, then a quick go with 1200 to really smooth it out. This was slightly overkill - if you can't see scratches when it's wet, you won't see them when they're covered in epoxy.

This was the technique I mostly used:

Flatting round pieces by comatus1649, on Flickr
For flat bits you could use a block, but there aren't many flat bits on a bike (bottom of the BB shell is the only example really, unless you're doing some ambitious aero nonsense). Excuse the dodgy photo, I don't have three hands and had to wedge my phone between the chainstays ...

It's ok if you get down to fabric in places, but that's a sign you need to back off because once you start damaging the fibres you'll change the final appearance. It's hard to describe what exactly it looks like when you go too deep, but you'll see when you do it - the white high points, instead of looking like random lumps and drips, will look like tiny rectangles at right angles to each other.

Once you've washed off all the dust (and seriously do wash it off, in the shower and everything, because otherwise those little bits of carbon and epoxy dust will fuck it up), it will all look kinda ruined:

After being flatted by comatus1649, on Flickr
But that's alright - it's much flatter now which will make for a good final result. The reason we have to put all that epoxy on and then take it all off again is that there will be some bits of fabric that are maybe 2 or 3mm below the general layer of the part, and you want the surface to be as free from pitting as possible.

This is also a good opportunity for a few sanity checks - will the headset cups fit, for example, or is the joint overbuilt? Does the carbon extend over the edge of the BB shell, or too far onto the dropouts? You can fix all this later if you have to, but it would be nice not to have to.

Now for one final coat of epoxy. I applied this with paper towels to get a really smooth, thin, run-free surface. After this cured, I had a pretty good finish, dust nibs and the odd concealable pit aside:

After final very thin coat of epoxy by comatus1649, on Flickr

Next step is to polish this up with 1200grit, and think about lacquer ... This is the third week in a row I've said this, but I really do think it'll be done this week.