Should the underside of a lead keel be coated with epoxy?
My sailing yacht has a lead keel, and I'm repainting the bottom while she's ashore. Should the flat underside have epoxy primer as well as antifouling, including the patches where the keel rests on blocks?
Answer
Yes, the underside of an exposed lead keel should normally have a suitable epoxy primer beneath the antifouling, just like the keel's sides. If the existing epoxy is firmly attached, keep it and repair damaged patches rather than stripping everything back to bare lead.
Check the existing coating
Wash off slime and loose antifouling, then inspect the underside, particularly around the supporting blocks and any grounding scars. Establish whether you have exposed lead, sound primer or a coating separating from the metal. Each needs different preparation.
Bare lead is usually dull grey and relatively soft. Its surface can develop a pale, powdery deposit, while old primer may form a distinct, harder layer beneath the antifouling. Colour alone is unreliable because primer colours vary.
- Sound primer: clean and abrade it as required by the coating system, then apply compatible antifouling.
- Small bare patches: remove loose edges, prepare the exposed lead and rebuild the primer locally.
- Widespread peeling: remove the failed coating and investigate preparation or compatibility before recoating.
If the ballast is completely enclosed in fibreglass, you are coating the outer laminate or gelcoat. Do not grind through it looking for lead.
Why lead needs epoxy
Epoxy primer provides an adherent base for the underwater paint system and helps keep water away from the metal-to-coating interface. Lead does not rust like an iron keel, and solid lead does not suffer fibreglass osmosis. Neither is a reason for applying epoxy.
The practical problem is adhesion. Lead develops surface oxides, and coatings applied over loose oxidation, contamination or poorly attached paint can peel. The underside also gets scraped during groundings and compressed against storage blocks, so its coating often needs more attention than the sides.
Use a marine epoxy primer whose current technical instructions explicitly cover lead or specify suitable preparation for it. International Interprotect and Hempel Light Primer are familiar products to investigate, but check the particular product's local instructions. Do not assume every epoxy primer is interchangeable. Fairing filler restores shape; primer provides the coating foundation.
Prepare exposed lead patches
For a local repair, follow this sequence:
- Remove fouling, salts and contamination, and let the surface dry before mechanical preparation.
- Scrape away loose paint and feather the surrounding sound coating without unnecessarily removing lead.
- Prepare the exposed metal using the primer manufacturer's specified method. Where abrasion is specified, approximately 80-grit abrasive is a common starting point.
- Remove preparation debris and apply the first coat promptly within the specified preparation window, before fresh oxidation compromises adhesion.
- Apply the remaining coats to achieve the specified dry-film thickness, then apply compatible antifouling within its stated overcoating window.