Recently on one of the spinning lists I subscribe to, there was a thread about onion skin dyeing. It was eventually mentioned how onion skins don't make the most permanent dye and a few people recommended pre mordanting the wool before dyeing it with the onion skins. One person though, claimed that this would make the dye not only more washfast but also more lightfast.
I've always thought that the only ways to keep dye from fading due to light exposure is to either keep it away from light or to keep it it away from oxygen, both of which would be pretty hard for a garment that is designed to be used. Could mordanting really improve light fastness?