Having assisted with a repair to a glass fibre glider fuselage, it is worth noting that it considerable effort was taken to make sure each layer of glass fibre matched that in the original fuselage in both type density and weave of glass clothas well as the direction of the fibres. The repairer in this case was a graduate of one of the German Akaflieg universities where they tend to know their stuff about composites.
The technique used was to mark both the material to be removed and the remaining structure to ensure correct alignment of the removed portion, then to burn off the resin on the edges of the removed portion to expose the fibres in the glass layers to ascertain the fibres' orientation.