Narrowboat Blacking Explained: Is Mine Overdue?
A steel narrowboat hull needs blacking roughly every 2-3 years if coated with bitumen, or every 5-6 years with 2-pack epoxy. If you don't have a dated invoice or survey note showing when it was last done, assume it's due.
What is blacking?
Blacking is a protective coating applied to a steel hull below (and just above) the waterline to slow corrosion. The boat comes out of the water (via crane or slipway) where the hull is pressure-washed, taken back to sound steel, treated for rust, and repainted. A thorough job applies multiple coats below the waterline (usually 2-5 coats), and the boat typically stays out for several days to dry.
How much does blacking cost?
Budget £800-£3,000 for a professional bitumen blacking on a typical narrowboat, including dry dock fees. Price scales with boat length, the number of coats, and yard rates in your area. 2-pack epoxy costs significantly more per application (often several times the bitumen price, especially if an old bitumen coating must be shot-blasted off first) but its 5-6 year lifespan can make it cheaper per year of protection.
While the boat is out of the water, it's the natural time to renew the sacrificial anodes (typically around £200 per pair fitted, replaced when roughly half-wasted) and to commission a hull survey if necessary.
How do I know if the blacking is overdue?
Work through these checks:
- Paperwork first. A dated boatyard invoice or a hull survey noting the coating type and date is the only reliable answer. No record usually means overdue.
- Do the maths. Bitumen applied more than 3 years ago, or epoxy more than 6 years ago, is past its interval.
- Look at the waterline. Rust streaks, orange pitting, flaking black paint, or heavy marine growth along the waterline all suggest the coating has broken down.
What happens if I skip it?
The steel corrodes faster. Narrowboat hull sides are commonly only 6mm thick with a 10mm baseplate. And once localised pitting takes underwater steel below about 4mm, the boat risks becoming uninsurable or needing overplating. A missed £1,000 blacking can quietly become a £10,000+ problem.
Should I choose bitumen or 2-pack epoxy?
Stick with what's on the hull unless you're ready to pay for a full reset. Epoxy won't bond reliably over old bitumen, so switching means shot-blasting back to bare steel first. If the hull is already epoxy-coated (common on newer boats), maintain it as epoxy.
Do GRP boats need blacking?
No. Fibreglass doesn't rust, so GRP cruisers skip blacking entirely. Instead, check the gelcoat below the waterline for blistering or osmosis, and maintain antifouling as needed.
Buying a boat? Blacking history is key.
For buyers, the blacking record is one of the best single indicators of how a boat has been cared for. Always ask when it was last blacked, with what product, and by whom. A seller who can't answer or show the invoice is telling you something about the rest of the maintenance too.