If it looks like battery acid damage you could try to neutralize the acid with the baking soda, then clean up the baking soda and allow the fiberglass tiem to dry out to see if it re-hardens.
If it’s not battery acid and you cannot determine what else might have caused the damage, you’ll probably have to bite the bullet and cut out the damage piece and glass in a new piece of fiberglass.
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster
If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)