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5 hours ago, Dr Bob said:

Jim, you may have been just lucky.

Because a coating is water based does not mean it is tolerant of a damp surface. You must follow the manufacturers guidelines and only apply to steel 3 deg above the dew point. Perhaps you did.

You are giving people incorrect info on a rotoblaster. Yes, to you it may be almost as good as sandblasting but how are you 'sandblasting'? Lets get rid of 'sand' as its not allowed any more. 'Grit blasting' is done to a standard ie SA2.5 required by most paint manufacturers for ultimate performance. This gives a cleanliness and profile standard. You will not achieve that with a rotoblaster. Maybe, just maybe, you could get near the clean but surface profile? .....and how long will it take? Mechanical cleaning is much more diffcult than grit blasting. Ok on small areas if you can get the surface profile right.

For applying blacking (not 2 pack epoxies), water jetting is fine as the coating sticks enough to last 2 years so you dont need to get to an SA 2.5 finish.

 

eta. The ability of Keelblack to be applied to a damp surface is not because it is water based - it is because the coating has been formulated to stick to less than perfect surfaces. OK for a 2 year life.

 

eta more: Just checked the application sheet and they dont ask for a temp above a dew point so ok to apply it cold. Having spent the years I have in paint formulation, I would'nt apply it at the temps they say!!!!

Ha! Best to read the spec sheet first. As for the damp surface, there's no way you could put an oil/solvent based paint on a damp surface, but a water based paint? It will marginally be diluted but otherwise what's the problem. It's far more likely not to be a problem and when doing the research I read that some boatyards are now using keelblack because it doesn't have to be bone dry. 

As for age related stuff, the keelblack has been on nigh on 2 years, it's only suffered where I did some ice breaking (first steel boat after grp so it just had to be done?) and where there has been the odd scrape on stonework. I've also inspected below the waterline a couple of times, the pound at my mooring goes up and down like a nores nickers. It's stuck fast where its not been scraped/bashed off. 

As for the rotoblaster, where I was removing old blacking from "new" steel plate it was left clean and shiny, also did a good job on the rusted bits, sufficient to my eye and for my needs. Grit blasting isn't allowed in the dry dock in Hebden Bridge. I intend to use the rotoblaster on the slightly rusted  bits, followed by a coat of fertan as recommended by keelblack.

Someone said earlier that there's no proof that keelblack is better than others, however the converse is also true, no proof they are better than keelblack. 

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14 minutes ago, Jim Riley said:

 As for the damp surface, there's no way you could put an oil/solvent based paint on a damp surface, but a water based paint? It will marginally be diluted but otherwise what's the problem. It's far more likely not to be a problem and when doing the research I read that some boatyards are now using keelblack because it doesn't have to be bone dry. 


?

 

yes, I should have read the spec sheet first!

.....but painting on damp surfaces is possible but it is not about oil or waterbased paints. I wont go into the technical detail here but a barrier coating (as these are) has to 'stick' to the surface. The fact that Keelblack is water based makes no difference to the adhesion of the chemical in the coating (in this case a bitumen type molecule) which is highly hydrophobic (ie doesnt like water). To have a damp tolerant coating, you need to physically displace the water from the surface before the active chemical can bond to the surface. Therefore, Keelblack will contain an additive that displaces that water. Its the same with surface tolerant epoxies which are solvent based. They dont like water but you can put in something to displace the water. The clever trick is finding something that will displace the water. Water based paints therefore are no better than solvent based paints. In fact I am surprised how a water based paint CAN displace the water efficiently enough. 20 years ago, solvent based paints were the bees knees. HSE has driven water based into the market but they have never performed as well as solvent based. Maybe they are better now....but I doubt it.

Another rule of thumb is that surface tolerant coatings are a compromise over proper coatings on well prepared surfaces. You take your chances. They are not that surface tolerant. On a job that only requires two years life however, anything goes and almost any coating should last the course.

I do have many years experience in the high performance splash zone coatings sector but just dont seem to be able to get worked up over the blacking of my boat. in 2017 we had it blacked with International stuff. I'll do it again this year with the same. Cant really be bothered about finding anything better as there is 8mm of steel or wotever and that is not going to disappear fast and the only other solution is grit blast to SA2.5 and two coats of epoxy.....or zinga first.

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Interesting topic for a newbie like me !

 

I was just thinking about when I take a boat out of the water and blacken the underside - sounds like I don't need to.

 

So, RUST needs air, wet/dry/damp conditions to happen. The baseplate is always submerged so rust shouldn't happen.

 

The sides of the boat can take impacts, get wet and dry, and can therefore rust, therefore protection and blackening needs to be done regularly.

 

Corrosion can happen because of *other things* - this could include being regularly plugged into shore power/having no galvanic isolator, chemical/electrical issues, etc.

 

This is what I am thinking in my head - is this a basic, but fundamentally sound summary ?!

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10 minutes ago, NewCanalBoy said:

So, RUST needs air, wet/dry/damp conditions to happen. The baseplate is always submerged so rust shouldn't happen.

 

Not quite right. Rust needs OXYGEN to occur.

 

But you're right about the baseplate being submerged so it can't happen. There is no oxygen in the water to support corrosion. If there was, the canal would probably be full of fish, wouldn't it!! 

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1 hour ago, Mike the Boilerman said:

 

Not quite right. Rust needs OXYGEN to occur.

 

Yes indeed - Oxygen.

 

Now blackening - I assume there are different ways to approach this.

 

You go to a place where they can get the boat out of the water and tell them 'Please blacken that boat' - disappear for a while, come back a couple of days later and then pay them. You still smell nice and they are happy.

 

Other way, somebody takes it out of the water - you use (hire ?) a pressure washer, does this get old bitumen off or just green smelly stuff ?, paint on new stuff yourself, you get caked in sh*t but your kind of happy as you've saved a few quid. 

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3 minutes ago, NewCanalBoy said:

Yes indeed - Oxygen.

 

Now blackening - I assume there are different ways to approach this.

 

You go to a place where they can get the boat out of the water and tell them 'Please blacken that boat' - disappear for a while, come back a couple of days later and then pay them. You still smell nice and they are happy.

 

Other way, somebody takes it out of the water - you use (hire ?) a pressure washer, does this get old bitumen off or just green smelly stuff ?, paint on new stuff yourself, you get caked in sh*t but your kind of happy as you've saved a few quid. 

I prefer "other way". Hebden Bridge dry dock, Bronte boats let me use a petrol driven pressure washer with rotating tip (it would skin a cat easy, shame there were none around to try) for free, though I did pay them to do a bit of welding. I wasn't caked in sh1t this time. First time the boat hadn't been blacked for 10 years and there was quite a crust of rust on the baseplate, I used a wire cup brush, got absolutely covered in old blacking and rust despite using a one piece paper suit, goggles and a face shield. 

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