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Blacking - Waterline Rust

We were surprised to find quite a bit of rust on the waterline of our narrowboat hull after the winter as the boat was only blacked 18 months ago and the anodes were checked and some renewed. It is important to get the boat blacked now or could the job be left to September? What treatment should be done to deal with the rust?

Asked by: Clive Dutson  | 10.14am, Thursday 2 May


WW says:

It really depends on what type of paint was used on the hull. If it was a basic paint like Bitumastic, two years between dry dockings is not unusual so you would expect to find some rusting after 18 months. The amount of corrosion also depends on how contaminated the water in your canal is and whether you you have removed any paint on the waterline by possible cruising through ice. Paint can also be damaged by passing boats breaking ice even when you are moored.
Anodes are more useful for protecting the underwater sections of the hull around them but would not have a great deal of effect on rust at the waterline.
Also, if you leave the boat plugged into the mains in a marina, you should have a galvanic isolator installed in the earth wire to prevent galvanic corrosion.
Without seeing the boat, I would have thought that you could safely leave the re-blacking until September as not much more is going to happen in four or five months.
Boats are normally pressure jetted to clean off the muck and grime and, if you are applying a basic finish, this will be all that is required before repainting. If there are still signs of rust, they can be removed with a powered abrasive disc or wire brush.
The long-term solution is to put a better finish on the boat but, in most cases, all the old finish will have to be removed first. The ultimate solution is two-part epoxy pitch but the boat has to be grit-blasted to produce a perfectly clean surface before it is applied. This is naturally expensive but, if you are planning to keep the boat for a number of years, it can pay for itself by reducing the number of dry dockings you have.
For more specialist advice, visit http://www.mainlinebuilding.com/ or ring 0151 920 6451.

Graham Booth  | 12.22PM, Thursday 2 May

Most corrosion to a boat is around the waterline, as its a oxygen and water rich environment, plus open to abrasion as well. it may well be that the blacking that was used did not adhere well to the surface to begin with, or that corrosion had already taken hold before the coating went on. Or, the thickness of blacking was insufficient (too thin or too few coats). Cheaper, basic bitumen may well only last a year or two before needing overcoating.
if you boat is quite new (say within 4 years) the mill scale that is produced during rolling the steel plate, can lift, taking the surface coating with it. This is why many boats are gritblasted when new- to remove this unstable film, plus provide an excellent surface for paint adhesion.
You could leave the boat until the autumn, but any unprotected steel will continue to corrode. One treatment you may consider (and is harmless if any runs into the water) is Fertan. The iron tannate solution has the consistency of thick cold tea, is water based, and converts the rust to insoluble iron tannat- which can be left uncovered for up to 6 months before painting. it also removes loose rust and is easy to apply by brush or roller. It can also form an excellent base coat before many conventional paint treatment systems- though not with some high performance epoxy substances.
Fertan can be bought from many outlets, or from www.fertan.co.uk directly. It certainly works well and can help hold corrosion at bay-0 it is used on many things from oil rigs to classic cars!
At the next blacking, it may be useful to check that the anodes are unpainted and are in good electrical connection to the hull (if they are bolted) or that they are not just held loosely against welds. Anodes help reduce corrosion but cannot stop completely waterline rusting, as the conditions are too great and atmospheric oxidation will happen. Also make sure that anodes fitted for freshwater use are magnesium, rather than zinc.

Mark Langley  | 12.25PM, Thursday 2 May

As a PS to my previous answer, you may also find the article about dry docking in the November 2011 issue of WW interesting. It discusses paint types, whether to do it yourself or get a boatyard to do it, how often it needs doing, and goes on to review a useful tool for removing rust from the hull.
Back numbers of Waterways World can be obtained by ringing 01283 742970.

Graham Booth  | 4.44PM, Thursday 2 May

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