Fixing ferro is something of a black art, but having built (and fixed) a ferro yacht I'd suggest the following (Verify this with a naval architect before proceeding. Make sure they sign off on anything that affects the structural integrity of the hull
1) Make sure the boat is out of the water
and supported so the area on which you are working is not loaded. Read the safety
for all material that you will be using and take the appropriate precautions, especially proper ventilation. Wear disposable overalls, gloves, goggles and hearing protection.
2) Totally degrease and clean the entire area. Remove paint back to bare cement or epoxy
primer over surrounding area. This includes the entire bottom of the bilge
. Try to get good access.
3) Chip back any oil
contaminated areas as far as practicable. Your guess is as good as mine as to how far this is. Don't do this while the boat is in the water
. Oil and seawater will damage ferro.
4) Allow the entire area to dry totally.
5) If the amount of concrete removed is very substantial degrease any contaminated reinforcement and replace any damaged reinforcement. Replace any damaged cement with good quality portland
cement, sand and pozzolan (Check for correct mixing ratio). Use a bonding agent to join new cement to old cement.
6) If the chipped back area is small fill with epoxy
7) Reprime the entire area with thin epoxy primer.
8) As the epoxy primer goes "cheesy" topcoat with good thick gooey (technical term - look it up) tar epoxy. If you're clean and happy after this you did it wrong - read the instructions and take care.
9) Get clean as best you can and reassemble boat. The tar epoxy wears off with your skin...