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orionb
orionb

Posted: Tue Oct 28, 2008 4:50pm

Post Subject: Hull Blacking

We have a four year old boat which was blacked 2 years ago with bitumen based blacking, it was power washed and then 2/3 coats of blacking applied. After 2 years our boat is now extremely rusty above, below and on the waterline and in comparison to other craft appears to have considerably more rust. Most boaters from our boat club have theirs blacked at the same place and by the same methods and appear not to have such problems with rust. Should we be looking at having the hull shot blasted, which would be very expensive or are there other ways we could tackle this. I would appreciate your thoughts on this and what you consider is the best way forward. Many thanks in advance for your advice. Frank Lovegrove

Tony-B
Tony-B

Posted: Tue Oct 28, 2008 8:43pm

Post Subject: Hull Blacking

Dear Frank With such a new boat and with little apparent attempt to disk the rusty areas off last time my first suspicion would be its mill scale that has been punctured so water and rust could creep beneath it so the rust lifted chunks of scale off complete with blacking. If this is so then another pressure wash, disking/grinding the rusty areas and re-blacking should be better, but not a total cure until the majority of the mill scale has fallen off. This is not particularly uncommon on new hulls unless they were shot-blasted when new or built from scale free plate to begin with. The next question has to be about your use of shore power. If you are using a shoreline connected to the boat's own mains circuits for long periods then this might indicate a lack of galvanic isolation on the mains earth. There is also a question about the type of coating (tar based, bitumen based, basic or modified) that was used and how it reacts to any pollutants in the water. Finally I have been hearing odd snippets about this sort of problem with a major manufacturer's blacking product but without hard evidence I can not even hint at them on a public forum like this. We must not ignore the possibility that the blacking was applied to a damp hull. Remember the typical blacking interval is between every two and four years. If this was my boat I would pressure wash, grind the rust and re-coat. I would take care to touch up any film punctures above the waterline and then keep and eye on what is happening. Tony Brooks

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