1. There are points where you transition from a single layer of cloth to two (or more layers), or where a single layer of cloth transitions to bare timber, (such as at the edge of a taped-seam), tend to be ragged and the edge, or selvege, of the cloth lifts up to form a ridge which is difficult to sand and fair;
2. The surface of the cloth is usually uneven and requires further filling with epoxy resin/hardener mix. The problem here, apart from time and labour, is that you need to ensure that subsequent layers are applied while the preceding coats are still chemically active. If not, you must remove amine blush and then sand the surface to provide a keyed surface suitable for a mechanical (rather than chemical) bond - commonly referred to as a secondary bond (rather than a primary bond).
In many circumstances, you can get around all of these issues by applying a layer of "Peel-Ply" over the epoxy/fabric layer while it is still wet. The peel-ply is simply a fine-weave fabric - a bit like shower curtain - which allows epoxy to wick through but does not adhere strongly to the wet epoxy. When the epoxy cures, the peel-ply is 'peeled' off to reveal a nicely filled fabric (due to the magic of capiliary attraction or surface tension, or something like that!). In the process, the amine blush is removed and the resulting surface is ready for the next coat - or it can be lightly sanded and then painted.
Here are a few pictures of a small job I did yesterday and the day before. It involved the application of three staggered layers of uni-directional carbon-fibre over the outer surface of a snotter attachment cleat on a carbon mast I'm setting up for a customer - it could have been anything, but this little job demonstrates the process quite well.
|The bare timber (Celery-Top Pine) snotter cleat epoxied to the forward face of the mast. The carbon-fibre mast had been sanded on the mating surface to provide a good gluing bond.|
|Next morning I peeled off the peel-ply|
|...to reveal a nice, well-filled surface|
|...which only needed to have the edges trimmed with a spokeshave or a sanding block.|