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Does a new hull need shotblasting? And source of steel?


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5 hours ago, Man 'o Kent said:

I'd like to have a little clarification here. Are we talking shot blasting or grit blasting?

Abrasive blast cleaning using grit abrasives. 
 

The standard grades of cleanliness for abrasive blast cleaning in accordance with BS EN ISO 8501-1[1] are:

Sa 1 – Light blast cleaning

Sa 2 – Thorough blast cleaning

Sa 2½ – Very thorough blast cleaning

Sa 3 – Blast cleaning to visually clean steel

 

We are talking about Sa 2½ prep. 

 

 

Edited by WotEver
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8 hours ago, PeterF said:

I have heard second hand that Colecraft may now have shot blasting facilities, they were apparently testing equipment 18 months ago. You may like to confirm this though rather than relying on what is now third hand information.

Thanks PeterF. That’s encouraging. I will bring it up with Colecraft when I start making enquires with builders

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I think that we are talking grit blasting, my boat builder talks about needing to clean out a residue of garnet after the boat has been blasted. To the layman it is all shot blasting even if it is grit blasting.

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16 minutes ago, PeterF said:

I think that we are talking grit blasting, my boat builder talks about needing to clean out a residue of garnet after the boat has been blasted. To the layman it is all shot blasting even if it is grit blasting.

I'd think more people call it sandblasting than shotblasting, even though it isn't that either.

 

(Silicates shatter making dangerous dust - very fine particles that gets in lungs.  Powdered garnet is the stuff they use, as you rightly say.  Looks like sand but isn't!)

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The SA system is a classification of cleanliness as stated by @WotEver above. How rough the surface will be depends upon the blasting medium. The paint systems referenced in the article above don’t specify anything beyond the cleanliness but I would have thought they are better applied to a rougher surface.

 

Bear in mind that in general narrowboats will probably not be prepped in a purpose built blasting facility, they just aren’t built in enough volume to make that feasible for most - if not all - fabricators. The Tyler Wilson philosophy is presumably to do what they can do well rather than to do a theoretically better treatment badly. They then apply a coating that is compatible with what they can achieve. This works for much general steel fabrication. As stated previously I think the only thing to avoid is anyone who isn’t prepping in accordance with the requirements of the coating manufacturer but still passes off their higher tech coating as a selling point.

 

I’ve written paint specs for large fabrications and been involved in maintenance painting of bridges - although I’m a generalist rather than a specialist on the subject - and I think to be insistent on blasting of hulls for narrowboats is a bit on the side of overkill, not least because even the best coatings will be short lived compared to many industrial applications. The reasons for doing it are probably more about piece of mind of the owner than they are about economics. There is no absolute right or wrong. It’s a personal choice. I think to a degree we’re all making a decision on something we don’t fully understand.

 

JP

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12 minutes ago, Captain Pegg said:

The SA system is a classification of cleanliness as stated by @WotEver above. How rough the surface will be depends upon the blasting medium. The paint systems referenced in the article above don’t specify anything beyond the cleanliness but I would have thought they are better applied to a rougher surface.

 

Bear in mind that in general narrowboats will probably not be prepped in a purpose built blasting facility, they just aren’t built in enough volume to make that feasible for most - if not all - fabricators. The Tyler Wilson philosophy is presumably to do what they can do well rather than to do a theoretically better treatment badly. They then apply a coating that is compatible with what they can achieve. This works for much general steel fabrication. As stated previously I think the only thing to avoid is anyone who isn’t prepping in accordance with the requirements of the coating manufacturer but still passes off their higher tech coating as a selling point.

 

I’ve written paint specs for large fabrications and been involved in maintenance painting of bridges - although I’m a generalist rather than a specialist on the subject - and I think to be insistent on blasting of hulls for narrowboats is a bit on the side of overkill, not least because even the best coatings will be short lived compared to many industrial applications. The reasons for doing it are probably more about piece of mind of the owner than they are about economics. There is no absolute right or wrong. It’s a personal choice. I think to a degree we’re all making a decision on something we don’t fully understand.

 

JP

I agree with most of the above but you need to remember that the coating manufacturers formulate their coatings -well 2 pack epoxies anyway - and test them on 'standard ' surfaces.......so that is how they predict life time. Most companies will test them on grit basted steel to Sa 2 1/2 and that will be what they recommend the painting is done to. The roughness of the surface is important to barrier coatings (rather than passivating coatings) to achieve the ultimate adhesion. Any lesser surface will be a compromise despite everyone pushing their surface tolerant epoxies which may work to a point but certainly not as well as grit blasting. 

We are in the process of getting a new boat but that will be blacked rather than 2 packed. Painting outside in the UK (for 2 packs) is not easy and really should be done indoors by guys who know what surface is required, to avoid flash rusting and to apply the coatings at the right temperature.

Grit blasting replaced sand blasting ...that did the same job....back in the late seventies / early eighties due to the HSE issue Pete described above.

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On 09/06/2020 at 20:47, Alan de Enfield said:

If you don't it is false economy, the paint will stick to the scale and when the scale falls off the paint goes with it.

 

 

I agree with this but I also think that mill scale has got a bit of a bad press. ?

 

My hull was/is completely covered in mill scale including the topsides. This was 15 years ago. The mill scale has never fallen off and I don't think it ever will.

 

When I epoxied it 5 years ago I had the hull grit blasted up to the top rubbing strake to get all the bitumen off. The scale was so well bonded the guy doing the work had a hard time getting it off. In some places the grit just etched the mill scale.

 

Scan0003.jpg

Edited by blackrose
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10 hours ago, Man 'o Kent said:

I'd like to have a little clarification here. Are we talking shot blasting or grit blasting?

 

Shot blasting removes loose material like millscale by percussion.

Grit blasting removes loose material by abrasion.

Shot blasting peens, (hammers), the surface thus very slightly compressing it in the process and possibly embedding particles of millscale/rust into the surface.

Grit blasting act in a very similar way to a grinding wheel insofar that the sharp grit particles actually cut the surface. Anything poorly attached will be knocked off but on sound metal the grit "picks" at the surface, (individual particles do not have enough energy to do more), but do raise tiny burrs.

So my take on the two processes is that shot blasting tends to smooth the surface, grit blasting roughens it. Given that the surfaces so prepared are to be painted/coated which would the membership rather have?

Good job no one said sand blasting

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2 hours ago, Captain Pegg said:

 

Bear in mind that in general narrowboats will probably not be prepped in a purpose built blasting facility, they just aren’t built in enough volume to make that feasible for most - if not all - fabricators. The Tyler Wilson philosophy is presumably to do what they can do well rather than to do a theoretically better treatment badly. They then apply a coating that is compatible with what they can achieve. This works for much general steel fabrication. As stated previously I think the only thing to avoid is anyone who isn’t prepping in accordance with the requirements of the coating manufacturer but still passes off their higher tech coating as a selling point.

 

I’ve written paint specs for large fabrications and been involved in maintenance painting of bridges - although I’m a generalist rather than a specialist on the subject - and I think to be insistent on blasting of hulls for narrowboats is a bit on the side of overkill, not least because even the best coatings will be short lived compared to many industrial applications. The reasons for doing it are probably more about piece of mind of the owner than they are about economics. There is no absolute right or wrong. It’s a personal choice. I think to a degree we’re all making a decision on something we don’t fully understand.

 

JP

Wise advice JP and puts things in context. Along the way of my career I have been involved with the development of floating offshore oil production vessels (FPSOs) where a hull coating needs to last for 20 years. Getting an FPSO into dry dock is somewhat more of a rigmarole than pulling a narrowboat out, so the rationale is different. Though if you could get away with 20 years on a 'domestic' boat, that would be pretty awesome...although your average narrowboat probably has more scrapes in service than an offshore FPSO...

 

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On 09/06/2020 at 21:15, matty40s said:

The reason for gritblasting is to remove millscale. 

If you dont do this, the first coat of  blacking/two pack or anything else will not adhere properly and be coming off in sheets within months.  I have seen many boats from some builders with this problem

 

 

My blacking was just painted onto the millscale and it never fell off. I blacked it 3 times in the first 10 years until I eventually had it gritblasted.

Edited by blackrose
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On 10/06/2020 at 11:32, rustynewbery said:

18 months after launch, not a jot of the 2 original coats of Comastic was left on our Colecraft hull below the water line.  A well respected marine surveyor reported that it was caused by mill scale.  He recommended shot blasting and 2-pack. 

 

 

Sounds like poor prep from a boat builder who is often lauded on this forum.

15 hours ago, Dr Bob said:

Painting over mill scale is just plain daft. Mill scale is very difficult to remove from the steel  surface when you want to ...ie when wanting to paint....but it just falls off when you dont want it to come off ....ie with big swings of temperature above and below zero. One way of removing scale is to leave the steel outside over winter and watch it fall off.

 

 

How come it's never fallen off my boat's topsides in 15 years? I've just got around to painting the top sides for the first time! 

Edited by blackrose
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28 minutes ago, blackrose said:

 

Sounds like poor prep from a boat builder who is often lauded on this forum.

 

How come it's never fallen off my boat's topsides in 15 years? I've just got around to painting the top sides for the fist time! 

A combination of good fortune and good mechanical surface preparation is the most likely. The occurrence or non-occurrence of a phenomenon in a single or even a few cases is pretty much evidence of nothing.

 

It did occur to me that why @matty40s said he encountered many boats from some builders that lose their coatings early in life is that there are yards who are failing to achieve the requisite standard for mechanical preparation which would generally require the removal of loosely adhering but not well adhered mill scale.

 

I routinely see steel sections that retain mill scale after many years of life, although in those instances it’s presence is detrimental to nothing. It isn’t a given that it will all fall off but if the surface is poorly prepared before being coated it’s more likely.

 

JP

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2 hours ago, Captain Pegg said:

A combination of good fortune and good mechanical surface preparation is the most likely. The occurrence or non-occurrence of a phenomenon in a single or even a few cases is pretty much evidence of nothing.

 

It did occur to me that why @matty40s said he encountered many boats from some builders that lose their coatings early in life is that there are yards who are failing to achieve the requisite standard for mechanical preparation which would generally require the removal of loosely adhering but not well adhered mill scale.

 

I routinely see steel sections that retain mill scale after many years of life, although in those instances it’s presence is detrimental to nothing. It isn’t a given that it will all fall off but if the surface is poorly prepared before being coated it’s more likely.

 

JP

Agreed.

Sometimes it sticks well, sometimes it doesnt. Painting over it is a risk. In the Oil industry, our work on coatings was always specified to remove the mill scale as downtime to repaint was just never going to happen. If you spend £6-10K on a 2 coat bottom paint job, then I wouldnt take the risk. Hitting it with a hammer often works.

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