Blacking -again!!

[jddevel]

Currently got what I`d call a "flash" coat of Intertuf 16 on my sailaway but have decided to book with Debdales to have it hot metal sprayed late 2019 - they are booked up until then. So having read the reviews of Intertuf and some of the problems encountered what is recommended that I can put on until late 2019 as a "holding coat". Part of Debdales process is to grit back to bare metal.. Are there other products compatible that members would recommend so optimistically I can keep corrosion/erosion at bay until then and provide a perfect base for their work. Thank you.

[ditchcrawler]

Don't worry about  providing a perfect base for their work. They are going to blast off whatever you put there. Depending on how the hull was prepared before it was blacked it may or may not all fall off before 2019 anyway, plus anything you put on top

[DHutch]

When did the boat go into water? What does the coating look like now? Do you have a DIY dock nearby?

 

Daniel 

[jddevel]

6 hours ago, DHutch said:

When did the boat go into water? What does the coating look like now? Do you have a DIY dock nearby?

Never been in the water. It`s a sailaway nearing completion. Due to be in the canal middle August at the latest-I hope. Apart from one small area where the Intertuf appears to have what I believe may be grease beneath it the remainder appears intact although as stated appears simply to have had a single spray coat. It has been open to the elements since at least last September and can easily have further attention with another coating or two.

[DHutch]

Ah right. So assuming it's a thin as it looks, slap another coat on, should do two years as well as any other? One or two all over, and an extra one at the waterline?

As said, if it's painted on the scale odd bits might fall off anyway, but two years shouldn't be too bad. You can always heal it over and redo the waterline while afloat if it's getting bad halfway through the two stopgap years.

 

Daniel

[jddevel]

8 hours ago, DHutch said:

You can always heal it over and redo the waterline while afloa

Sounds like an excuse for a belated on board launching and naming party. Could roll the barrel across the roof to get the participants to co-operate. Then that causes another problem how to not to disturb the sediment!!!

[DHutch]

Please don't spoil good beer for protecting the waterline!

I have not done it myself but if you strap a 15ft length of timber at right angles extending over the side you won't need much weight to get a good heel!

Daniel

[WotEver]

I've also read on here about it being done with a couple of strap wrenches under the boat and attached to the far studs. Tightening them up will heel the boat securely, with the boat remaining stable until you release them. 

[Chewbacka]

I tried that and gave up as there was nothing on the bank side I trusted to take the load.  In the end I put everything heavy on the same side including some ballast weights I have hidden in lockers etc and managed about 7 degrees of list, which was enough for what I wanted.  I guess if I had done both I could have probably managed maybe 10 degrees, which is a fair bit.

[jddevel]

Have decided to give Keel Black "a go". It will be three coats on the Intertuf 16 and see how it survives between now and late 2019 ( assuming I do as well!!!)

[Bargebuilder]

But should you be considering hot metal zinc spraying? I would suggest not of you have a steel hulled boat that is to be used primarily in fresh water as opposed to sea water.

The reason for chosing this system as stated by some suppliers in their promotional material is: 

"How is all this relevant to the narrowboat owner? Well, we’re used to having to fit sacrificial zinc anodes to help protect a narrowboat’s hull from pitting, but we also know how localised this protection can be. Metal spraying effectively turns the entire hull into an anode. This means that when the blacking starts to wear or is damaged by underwater obstacles, the steel is still protected and the risks of pitting are substantially reduced."

The first problem with this statement is, that zinc is not recommended for anodes on boats in fresh water, instead, aluminium or magnesium is used. The main reason for this is that in contact with fresh water, within a few months, the surface of the zinc is made completely inactive by a build-up of zinc hydroxide, rendering it useless in terms of anodic protection. Magnesium gives excellent protection in fresh water and aluminium gives an acceptable level of protection in both fresh and salt water.

Even if the boat was used in salt water, if it has resided in fresh water first, then the layer of zinc hydroxide build-up creates a barrier that subsequently prevents the sprayed zink from providing any anodic protection.

In addition, it is often recommended that the sprayed zinc surface is then blacked or epoxy coated. This will also prevent it from giving anodic protection.

Another problem, is that if you fit magnesium anodes, the magnesium will not only be protecting the propeller, the prop shaft and the steel hull, it will also be protecting the hot zinc spray that you have just had sprayed on.

Whatever you coat your hull with, you cannot avoid fitting the recommended number of anodes and you must used Al or Zn in fresh water. The more exposed metal that your anodes have to protect, the heavier they need to be for any given period of protection, so achieving a really good coating of your steel hull is key to making your anodes last longer: Hot zinc coatings won't help. For anodes to work, it is imperative that the anode itself and the metal object that it is protecting remains as bare metal. Aluminium out-drives (z-drives) for instance will be protected by some sort of paint finish, but a magnesium anode will protect any parts where the paint is missing.

Having established that hot zinc spray provides no protection as an anode in fresh water, then its only use is as a protective coating, but is it the best available, especially considering the price and that you still are recommended to overcoat it?

It is true to say that a zinc spray should give protection from oxidation (rust), but so will paint!

Zinc spray will give no protection at all from electrolytic erosion, the sort that can produce serious hull thickness reduction due to stray electrical currents; particularly when the boat is connected to 'shore power', whereas, a good hull paint will.

The yard will say that any hull paint will erode away or be scraped off in time, which is correct, but just look at the digs and scrapes in the actual steel of any narrow boat! It is wrong to pretend that any surface coating, even zinc won't be heavily damaged during cruising and some epoxy coatings are incredibly hard and resistant to abrasion.

One huge disadvantage of zinc coatings, whether sprayed-on or proper hot-dip galvanising, is that it is a surface to which it is very difficult to apply other coatings: Just look at most gloss paints and read about the special metal etch primers which are needed to make them adhere properly. You often see galvanised garage doors with flakey paintwork for this very reason. 

Any paint finish or two-pack epoxy coating is only as good as the surface preparation beneath it. If a steel hull is grit blasted first, that is perfect preparation, but if you then hot metal spray it, it will be rendered shiny smooth again and a very poor surface for subsequent coating. If an epoxy paint coating is to be used, it alone will provide full protection against corrosion (rusting) and galvanic and electrolytic erosion, without the need for zinc spraying. The zinc coating will only reduce the adhesion of the subsequent paint job.

There are some incredible two pack coatings that are used in industry, but available to informed members of the public. Materials that are used to protect oil rigs, cooling water intakes for nuclear power stations, balast water tanks in ocean going tankers etc. and I have used one of them on my my barge. The material is called glass flake epoxy and actually contains flakes of glass for superb hardness and resistance to scratching and abrasion. It is best applied to blasted steel, but in conjunction with its surface tolerant primer it can be applied to gingered wet steel such as you are left with after wet-blasting. It can be applied by roller, brush or airless sprayer and is a very easy diy job.... if you like jobs done properly that is!  What's even better is that it sells for around £12 per kg so about £1,500 to £2,000 for a 60ft narrow boat. 

The manufacturer I used only supplies the stuff in grey, so a cosmetic coat of something black over the top might be required for visible areas. The glass flake epoxy itself will provide excellent protection for 15 years, if you don't scrape it all off as you bounce through tunnels!

I can see no point in hot metal zinc spraying, especially since there are arguably better and cheaper materials available and no waiting list for application.

I do have first hand experience of applying glass flake epoxy onto wet gingered steel and I can report that after 8 years it is still exactly the same as the day it was applied, but less the shine. Still providing complete protection against rust and because the surface of the steel is 100% coated, the anodes are hardly reduced at all because they have nothing to protect. Before you worry on my behalf, I have checked for electrical continuity between the anodes and the metals they are protecting and they are doing their job. 

 

 

 

 

[jddevel]

My Oh My, One question asked and answered and then along comes another contribution that once again throws me into a confused state of mind as to what`s best. More research advice needed.

[system 4-50]

12 hours ago, Bargebuilder said:

and you must used Al or Zn in fresh water.

??

[Bargebuilder]

Sorry, slip of the keyboard, I meant to write Al or Mg. Aluminium or Magnesium.

Further research on my part has revealed that 'hot zinc spray' is 10 to 15% porous! This information comes from 'Metallisation' the only UK manufacturer of thermal spraying equipment, as I understand it. If this is the case, then for total immersion one would be reliant on a good coating to keep the water out. This rather renders the sprayed zinc as unnecessary. Just go for the best possible coating and save yourself a fortune!