I've heard that there is an epoxy product that can be used to repair rotting exterior wooden window sills. Can you give me the product information?
You're correct - epoxy is the right product. Before you tackle a repair, though, evaluate whether repairing the problem makes the most sense. If most of the wood is intact and just one or two areas are rotted, carve that area out, fill, sand and repaint it. But if the entire sill is rotted, it's wiser to replace it.
If you do take the repair route, use a good quality two-part epoxy. One designed for this purpose is made by Advanced Repair Technology Their site also has excellent step by step instructions on accomplishing the job.