Get to the bot­tom of spots on hull, bro­ken glow plug, con­den­sa­tion and rud­der thick­ness

Canal Boat - - This Month -

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?


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.

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