I would not use glass...
If you bump the keel and water and rusts starts underneath, it will spread and then lift in sheets. It will be harder to grind and repair next time.Just grind to shinny metal, apply epoxy within the hour, than mix filler with the next coat. Aply the second coat while the prvious one is still tacky or else you need to sand. If you only have spots, or areas, you don;t have to do the whole keel. Use an angle grinder and grind the area until you find non rusted metal, then same as above.