Get to the bottom of spots on hull, broken glow plug, condensation and rudder thickness
QHaving not had any problems before, we had our boat blacked in September 2014, but noticed in August 2015 that the hull had orange spots (‘chicken pox’, as I described it). We contacted the people who had blacked it and they came out and looked at it in December 2015; they agreed there was a problem but couldn’t understand why it should be. However they reblacked it at half price in April 2016. Four months later, the problem returned.
One boatyard has told us it was mill scale, another that it was bad preparation and we would need to get it grit blasted. Any advice?
ELIZABETH MUNNERY, via email
ATONY REPLIES: Mill scale is very difficult and time-consuming to remove without grit blasting. Any mill scale left (more likely with a newer boat) will lift off with any coating, which is a reasonable explanation in the absence of any further information.
How was the hull prepared and how was blacking applied? Ideally the hull would have been pressure washed, then any rusty areas sanded/ground back to bare metal and any remaining blacking scraped to ensure it is still adhering well.
The first coat of blacking should be relatively thin and really scrubbed into all the pits in the hull. Subsequent coats can be of a thicker material and rolled on but care must be taken to ensure any pits are covered. Was the blacking was left to dry between coats and before putting back into the water?
If you have had a shoreline in use and you don’t have a galvanic isolator or isolation transformer, it could be the result of galvanic action, which produces pits in the steel – and if the blacking was not scrubbed into the pits it could cause the rust growths your photos show.
Finally it could be biological corrosion, where certain microbes actually eat the steel allowing the blacking to fall off. Remove the rust flakes and inspect the metal underneath as soon as the boat leaves the water. Bright shiny steel indicates possible galvanic action; rusty steel indicates physical damage to the blacking, mill scale, or historical galvanic corrosion; brownish slime tends to indicate microbial corrosion.
Personally I feel this looks like just a fact of life for boats with conventional blacking. Your photos look fairly normal for a boat blacked with a bitumen based product.
It’s often said that bitumen needs renewing every second year. Longer lives can be achieved by using a two-pack blacking, but the hull would need sand-blasting to ensure a clean rust free base. The ‘Rolls-Royce’ job would be to blast the hull, zinc spray or coat with a zinc-rich paint and then two-pack black.