Can a mirror be used instead of e.g. glass to get a good finish or will it be a problem with the epoxy?

Mirrors are just coated glass so will be fine as long as you use the non coated side

I have infused a lot of smaller pieces of CF on an old mirror, glued ( with epoxy .. ) on top of a thick piece of plywood ..  Works fine ..

A sheet of thick plate glass is better, for the simple reason that you can check your infusion from the "finish side" through the glass ..

I mainly use "EC Easy-lease" on the glass ..

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I find jigsaws are not the right tool for cutting thin laminates. The laminate vibrates too much and it's hard to get a nice cut. Especially when you've got some holes that are close to one another like on that piece, you may well fracture the fibres in those areas.