Rust spots below water line

[umpire111]

the chap in the boat moored next to mine informed me that after an inspection rust spots had been discovered below the water line on his boat. He was told that this was most probably due to stray currents from other boats moored nearby. His boat is coming out of the water to be re blackened even though it was only done last year. Today I noticed rust spots appearing on a rib also below the water line- seems a coincidence. I have a galvanic isolator fitted. Do I need to worry, I am moving the boat from that mooring in about 4 weeks.

Edited April 15, 2016 by umpire111

[ditchcrawler]

I bet you would have rust spots anyway after a year unless your boat was blasted and pained with 2 pack

[WotEver]

Galvanic isolators (or isolation transformers) are installed specifically to address the diagnosis supplied to the chap moored next you you.

 

Tony

[matty40s]

This is common on virtually all bitumen blacked boats moored in marinas. A galvanic isolator can slow this, but I havnt seen one yet that prevents it altogether after 2 years.

The application of the blacking may be suspect if it is bad after one year, blacked in winter with not enough curing time, or blacking over thinned, blacking not put on thick enough etc.

You may also need to test that your isolator is working effectively

Edited April 15, 2016 by matty40s

[Guest]

Ours used to get rust spots on the hull after 12 months of Bitumen, and there is no electricity in our marina. Two packed now and no spots after 2 years.

[ditchcrawler]

Ours used to get rust spots on the hull after 12 months of Bitumen, and there is no electricity in our marina. Two packed now and no spots after 2 years.

That's right, we have always had rust after 12 months and until 18 months ago never moored in a marina and even now no shore supply

[Geoff Taylor]

Hi, rust spots can vary greatly upon the quality of the steel used in boats, slight flaws in the rolling of the steel causes pockets of slag or air bubbles hence premature pitting.

Higher quality steel is inspected and certified against these flaws and given a certification code, Chinese steel used in boat building has been known to fail a hull survey inside 9 years old !.

For instance, our 38 year old 5mm hull on our Hancock & Lane has recently passed a hull survey with flying colours and is blacked with standard bitumen every 4 years.

She has never had a galvanic isolator fitted until I treated her to one last year.

[Bloomsberry]

My boat is in a marina

 

I've had it blacked with Bitumen 3 times in 5 years, twice by boatyards and once myself after a lot of prep.

 

Each time there was rust at the waterline within 12 months

 

My location in the marina is at the end of a jetty where diesel on the surface appears to get blown to, it's an end mooring so don't want to move from it.

 

The boat's coming out again in July and this time I'm planning on doing a DIY epoxy job & see how long that last for.

[ditchcrawler]

 

 

The boat's coming out again in July and this time I'm planning on doing a DIY epoxy job & see how long that last for.

My view is if you don't do it properly you will be wasting both time and money unless you have a yard with blasting facilities to hand.. I could be wrong but I don't know anyone who has had success like that.

[Bloomsberry]

My view is if you don't do it properly you will be wasting both time and money unless you have a yard with blasting facilities to hand.. I could be wrong but I don't know anyone who has had success like that.

 

Ok, but do you know people who have tried and failed and what their method of getting to bare steel was ?

 

I saw a Tercoo Blaster advertised a while ago so bought one and tried it out.

 

 

Does a great job of getting back to bare steel although is a bit time consuming so to do the complete hull isn't really feasible. I'm only planning on doing the waterline and any corrosion spots below the waterline for now depending on how much time I have.

 

It's got one chance and I'd rather throw my money at attempting a more lasting blacking solution than wasting it on another bitumen job.

[Sea Dog]

That's a handy looking tool! Can't help wondering if it would be easier to use and more effective in a grinder than on the end of a drill though. I'd be interested to hear how you get on with it and how the two-pack goes.

[Bloomsberry]

That's a handy looking tool! Can't help wondering if it would be easier to use and more effective in a grinder than on the end of a drill though. I'd be interested to hear how you get on with it and how the two-pack goes.

 

It comes on a spindle so is designed to fit in a drill, I think the rotation speed is important as if it goes too fast it can wear/damage the tips ?

 

I'll let you know how I get on, I'd be interested to hear from other people who have tried other alternative methods to gritblasting and how successful they were.

[WotEver]

Interesting. That appears to be identical (apart from the colour) to Cirrus's Perago Blaster. I bought one of those many years ago. It works well, albeit slowly.

 

http://cirrus-systems.orderfullfillment.co.uk/7-abrasive-disks

Edited April 16, 2016 by WotEver

[Murflynn]

 

It comes on a spindle so is designed to fit in a drill, I think the rotation speed is important as if it goes too fast it can wear/damage the tips ?

 

 

quote from Cirrus: Perago discs should NEVER be used in an angle grinder as the very high rotational speeds of this type of machine will cause severe vibration problems and the premature break up of the disk.

 

breaking up at angle grinder speed could cause you life-changing injuries.

 

 

 

 

It should be borne in mind that grit/sand blasting does not just clean the steel - it also produces a microscopically pitted surface profile that is ideal for bonding of paint.

 

Surface rust will appear within a few hours of blasting or of any other form of surface preparation unless you live in a desert with 0% humidity. High performance systems are compromised by surface rust. Blasting allows a complete hull (or a significant part of a hull) to be prepared and then primed within a few hours. Other forms of surface preparation do not.

 

I would agree that using an expensive epoxy-based system of steel not prepared by blasting is technically flawed. Whether or not the performance of the coating is affected is a matter of debate.

Edited April 16, 2016 by Murflynn

[Bloomsberry]

quote from Cirrus: Perago discs should NEVER be used in an angle grinder as the very high rotational speeds of this type of machine will cause severe vibration problems and the premature break up of the disk.

 

breaking up at angle grinder speed could cause you life-changing injuries.

 

 

 

 

It should be borne in mind that grit/sand blasting does not just clean the steel - it also produces a microscopically pitted surface profile that is ideal for bonding of paint.

 

Surface rust will appear within a few hours of blasting or of any other form of surface preparation unless you live in a desert with 0% humidity. High performance systems are compromised by surface rust. Blasting allows a complete hull (or a significant part of a hull) to be prepared and then primed within a few hours. Other forms of surface preparation do not.

 

I would agree that using an expensive epoxy-based system of steel not prepared by blasting is technically flawed. Whether or not the performance of the coating is affected is a matter of debate.

 

That's interesting, there is a boat yard I know of that epoxy's boats but prepares the steel by needle gunning.

 

I would have thought the finish on the metal afterwards would be similar to using the blaster ?

[ditchcrawler]

 

That's interesting, there is a boat yard I know of that epoxy's boats but prepares the steel by needle gunning.

 

I would have thought the finish on the metal afterwards would be similar to using the blaster ?

We use to use needle gunning for small jobs and lots of people doing it felt they were impinging debris into the steel, yes it takes off lose stuff, scale etc but its not the same as a blast finish.

[Murflynn]

 

That's interesting, there is a boat yard I know of that epoxy's boats but prepares the steel by needle gunning.

 

I would have thought the finish on the metal afterwards would be similar to using the blaster ?

you'd never complete enough in one morning to make it worthwhile applying the epoxy.

 

 

International Paint Guide:

 

Existing coatings: Removal by abrasive blasting is most effective; hand and power tool cleaning methods are also possible but much more labour intensive and best suited to small areas.

Rust: Should ideally be removed by abrasive blasting prior to coating but the extent of removal required will depend on the coating system to be applied. Hand and power tool methods are also possible but again, are more labour intensive and best suited to small areas.

[Claude]

This is common on virtually all bitumen blacked boats moored in marinas. A galvanic isolator can slow this, but I havnt seen one yet that prevents it altogether after 2 years.

 

 

A galvanic isolator or isolation transformer will only prevent galvanic corrosion. It won't slow or prevent normal corrosion.

Edited April 17, 2016 by Claude

[Murflynn]

 

A galvanic isolator or isolation transformer will only prevent galvanic corrosion. It won't slow or prevent normal corrosion.

what is 'normal corrosion'?

[valrene9600]

what is 'normal corrosion'?

Rusting along waterline?

[Murflynn]

Rusting along waterline?

what is the process that causes steel to rust at the waterline?

 

it's not a trick question - I think most of us understand about different metal immersed in impure water resulting in an electrolytic cell, but I for one am not fully conversant with 'non-electrolytic corrosion'.

[WotEver]

what is the process that causes steel to rust at the waterline?

Straightforward oxidising. Just like if you left a nail out in the rain - it turns orange.

 

Why at the waterline? Because diesel and other solvents floating on the water dissolve the bitumen allowing the steel to oxidise (rust).

 

Tony

[Bloomsberry]

you'd never complete enough in one morning to make it worthwhile applying the epoxy.

 

 

International Paint Guide:

 

Existing coatings: Removal by abrasive blasting is most effective; hand and power tool cleaning methods are also possible but much more labour intensive and best suited to small areas.

Rust: Should ideally be removed by abrasive blasting prior to coating but the extent of removal required will depend on the coating system to be applied. Hand and power tool methods are also possible but again, are more labour intensive and best suited to small areas.

 

Maybe , but you wouldn't offer needle gunning as a prep method for a professional epoxy blacking if it gave failed results, the buisiness wouldn't last long if it did. Blasting might be the most ideal method but there may be other methods that give satisfactory results.

 

Personally I'd like to hear opinions from people who have actually tried alternative methods of preping prior to epoxy blacking and hear what success/failures they have.

 

I find it hard to believe I am the only boater who's prepared to use hand tools to prepare a hull prior to applying epoxy blacking.

[David Mack]

Why at the waterline?

Because the rusting reaction requires iron, oxygen and water, all of which are present at the waterline.

[WotEver]

Because the rusting reaction requires iron, oxygen and water, all of which are present at the waterline.

Yes, I already know that. Or didn't you mean to quote me?

 

Tony