Sounds like you have a 50' steel-hulled Roberts with a gap between the ballast keel
and the hull. If this is the case, I would not fill the seam with anything solid like epoxy, it will end up failing and cracking out. You need a flexible material for this, I'd consider 5200. If it was a glass hull, filling with epoxy would be fine. On a steel hull
the dissimilar expansion and contraction rates of the ballast keel, the hull, and the epoxy will cause failure. If you insist on using epoxy, abrade until shiny, acid etch, and prime immediately before applying epoxy. Do not apply anything to steel or lead that is cold, it needs to be warm and dry.