PAINT BLISTERING OF STEEL HULL

[sidney]

I know it may be a bit of "how long is a piece of string", but would appreciate advice.

8 Year old, "quality" dutch steel cruiser, used only on non tidal, non salt water, well maintained, otherwise immaculate. Regularly lifted (2years) anti-fouled, would you expect significant paint blistering on hull? Does this suggest a production problem. Another problem? If blasted and repainted (six coats three primer, three antifoul I'm told!) is it likely to recurr. Is this boat likely to be more susceptible than others, now and in future? Does having the work done affect future value (downwards)? Thanks as ever for advice and comments. Best wishes <span style="color:blue"> </span>

 

[alienzdive]

any photos?

 

[sidney]

Sorry no pics. Small paint blisters below waterline in large patches over most of hull. Slightly more on port side. When punctured clear liquid and clean shiny metal revealed. /forums/images/graemlins/confused.gif

 

[FAITIRA]

Are the blisters under the steel primer?, right down to the steel? only under the watrline?.
I built a steel boat 44ftr.

 

[sidney]

Yes and Yes. Only under the waterline and yes under the primer down to bare steel.

 

[Mudplugger]

Might be worth your while having a look @ the Metalboat Society?? website... /forums/images/graemlins/blush.gif

 

[Bajansailor]

It sounds a bit like electrolysis which is creating these blisters. This is where you have stray electrical currents running through the hull, effectively creating (I think, I might be wrong) a type of impressed current cathodic protection system.
This could be the reason why the steel under the blisters is clean and shiny.
You will need to find the source of the problem before you go to the expense of blasting and re-painting, otherwise it will only return, no matter how good a paint coating you have.
Is any part of the electrical system grounded on to the hull?

 

[sidney]

Thank you for this. Not aware that any electrical system is grounded to the hull.

 

[PCUK]

Osmosis of the paint causes the blistering and liquid. The metal is still protected by the anodes. Antifouling will not protect the steel. Blast and epoxy coat to manufacturers spec' and then antifoul. Epoxy should last at least 20 years between recoats.

 

[FAITIRA]

Yes, sounds likely source of your problem.

 

[wotayottie]

there is no steel problem that I am aware of that will cause those blisters but I'm no expert. to my mind its most likely a paint problem, and painting steel for immersion is well established technology so no reason if it is well repainted that the problem should quickly re-occurr.

in your local area there is likely to be a company that supplies paint to steel fabricators. try the yellow pages. ask them to send their tech rep out to look and give you a schedule of how the boat should be re-painted.

 

[AndrewB]

[ QUOTE ]
there is no steel problem that I am aware of that will cause those blisters but I'm no expert. to my mind its most likely a paint problem, and painting steel for immersion is well established technology so no reason if it is well repainted that the problem should quickly re-occurr.

[/ QUOTE ]Yes, agree from my own experience with a steel yacht. I assume this blistering is occurring at or below the waterline, not above, and is not localised. Two possibilities spring to mind. How was the steel originally prepared for painting - was it degreased and an etch primer used to ensure proper primer adhesion? Has polyurethane paint been used in any underwater layer?

When repainting, I'd put a tie coat over 5 layers of epoxy beneath the antifouling. 2 coats of antifouling should be quite enough.

 

[andyball]

Hi.

Yes I get the same thing - clean shiny metal & a liquid in there.

One possibility is over-anodeing I'm told - perhaps imo aggravated by poorly adhering paint in the first place.

First time I saw it I was all doom n gloom, but aside from appearance & err resale time - they've caused no problems at all.

 

[gtuson]

I have steel hull ketch. It was grit blasted and epoxy/antifouled about 8 years ago. Had no problems at all with paint/corrosion until this year when noticed some blistering just as you describe. I note that your boat had been in fresh water. My boat had been on canals for 2 years prior to blistering. I did not use fresh-water/managanes anodes although did hang one over the side while in marina's over winter - electrically connected to hull....I suspect some sort of electolysis problem to do with being in fresh water....I'cleaned the paint back to primer and exposed metal beneath pin head sized blisters where possible and reepoxied....see what has happened when I look again over winter! Fingers crossed.

 

[FAITIRA]

I would bet it,s an electrolosis problem, I had a similar thing on the rudder of my Rob44, certain it was over annoding the rudder.

 

[sidney]

Hi Andy, Yes, was a doom and gloom moment, not sure I've still got over it. Many thanks to all for very helpful comments and advice and also PMs to which I will reply. Didn't think it was electrolosis, but will certainly check and also over anode'ing. Don't know whether I hope it was just faulty paint, trouble is will now worry in case the hull is being eaten away from under me....frightening thought. Best wishes Sid

 

[Bajansailor]

Hi Sid, one could say that the 'advantage' (for want of a better word!) of electrolysis is that it is a type of impressed current cathodic protection - hence the currents wandering about in the hull are effectively protecting it (to a certain extent) from being eaten - eg the blisters which were clean shiny metal underneath, ie not rusty.

However the disadvantage of electrolysis is that it really messes up the paint job below the waterline!

The above is simply based on experience with a steel boat we used to have - this electrolysis seemed to protect the steel on the outside, but the steel on the inside of the hull in the bilge still rusted away happily........

Ian Nicholson's book called 'Small steel craft' (I think that is the title - I had a copy, loaned it out, never got it back...) is very good, also I have heard good recommendations about a pair called 'Steel Boatbuilding' (I think) by Tom Colvin.

And Nigel Warren's book 'Metal Corrosion in boats' has got to be THE bible on this complex subject.

 

[Bajansailor]

Here are some quotes from Nigel Warren's book re 'Snags with Cathodic protection' :

"As with any galvanic cell the area of water around the cathode becomes alkaline - the greater the protective potential, the greater the alkalinity. Traditional types of paint (eg oil or lead based) are softened and blistered by alkali. Hydrogen bubbles may also be formed under the paint surface, depending on the porosity of the paint, thus lifting it off. The chances of either of these things happening is greater the higher the potential applied, so zinc anodes are the 'safest', while magnesium anodes or an impressed current system are worst. If a steel hull is over protected or has a porous oil based paint scheme the paint may 'saponify', but the bare steel thus exposed will be protected against rusting. The answer is to make sure that over protection is not being applied and then to use a more resistant paint. Best paints are the vinyl or coal tar epoxies.'

Could your boat be 'over protected'? I am a bit baffled here.....