Mill Scale

[Davebolli]

Hello

I'm looking to have a new boat built, but am surprised by the number of builders who leave Mill Scale on and paint over it albeit with two pack.

What is the general consensus of opinion on this question, am I being too fussy wanting the scale removed ?

Thanks

 

 

[Alan de Enfield]

Most builders will do whatever you want them to do - at a cost.

You get what you pay for.

[stegra]

My guess is that unless they have a closed of bay for shot-blasting, they are reluctant to have grit flying everywhere from open air blasting.

[Phil.]

No you are not being too fussy if you want a long lasting finish, even if using two pack.

[matty40s]

You aren't being fussy at all.

The amount of virtually new boats we see that have paint problems due to this cutting corner or nearly no blacking left on after 12 months means you are quite right in wanting this.......which should really be standard practice.

[Dr Bob]

Dont paint over mill scale. Given significant temperature cycling, mill scale will fall off the steel and so the paint will come off in sheets. Mill scale does not adhere to steel very well. Recommendations before applying 2 pack systems is usually to blast to SA2.5 which removes all mill scale.

I would read the application instructions on the paint and do what it says. If it says you can put it over mill scale then fine - but I very much doubt any paint company would be as daft as that.

[Dylan]

Our boat was blasted. I know others who didn't have them blasted, and have had no problems, but I was happier with it being done. The guy who two packed the hull said it was vital for this.

[MtB]

The original paint is still on my boat at 26 years old, and in very good nick. 

Does this mean mine must have been shot blasted?

[Giant]

You can get steel plate supplied already shot blasted and epoxy primed. We did this for our overplating and it meant there was much less prep to do before painting, after all the welding was finished. Obviously it will cost more than plate straight out of the mill, but might be worth it in time & problems saved later.

[Captain Pegg]

I agree that the likely reason the mill scale isn't always removed is the logistics of doing so. I suspect not all yards have the capacity; some may not do it on cost grounds. Some buyers want to build to a price after all.

The problem is that once the scale (which is a series of iron oxides of a different composition to the one we call rust) becomes cracked it will then be worked off through mechanical action over time. The shaping of the plates to form the hull will crack the scale hence the problems Matty sees. At source though I would say scale adheres to steel strongly hence it is a difficult process to remove it.

Agree with the view that if the hull can't be properly treated to remove scale then the use of pre-treated plate is wise.

If the scale isn't removed then the likely outcome is that with regular blacking it will be gradually worked off over time. If it's firmly adhered and soundly over painted it's probably fine but that's pretty much impossible to achieve over the whole of a new build hull.

JP

[Jess--]

my father always got his boats (and there were a few over the years) as bare steel, not even a primer on the outside.

then while the inside was fitted out the outside was left to rust for a year before being cleaned off and painted, the year of rust took care of the mill scale.

[cuthound]

I understand that Norton Canes Boatbuilders do the same to rid the shell of millscale. 

[nb Innisfree]

The paint on Innisfree's cabin sides and roof adhered well and stayed on the mill scale, it also stayed put on the hull sides for 2 years fit out but peeled off soon after launching followed by mill scale starting to flake off. We soon had it grit blasted which removed 90% but should have let it rust off for longer, alternative was shot blasting but as already stated not suitable in the open, in fact I believe it's illegal? 

[BEngo]

7 hours ago, nb Innisfree said:

The paint on Innisfree's cabin sides and roof adhered well and stayed on the mill scale, it also stayed put on the hull sides for 2 years fit out but peeled off soon after launching followed by mill scale starting to flake off. We soon had it grit blasted which removed 90% but should have let it rust off for longer, alternative was shot blasting but as already stated not suitable in the open, in fact I believe it's illegal? 

Not illegal, unless you are causing a statutory nuisance through noise or poĺution, say.  Could also be a civil problem.

 

N

[koukouvagia]

All the restoration work that we've had done on our butty has been fabricated from Hot Rolled Pickled and Oiled Steel.  This is more expensive but, so I'm told,  eliminates mill scale.  

 

[matty40s]

12 hours ago, Captain Pegg said:

I agree that the likely reason the mill scale isn't always removed is the logistics of doing so. I suspect not all yards have the capacity; some may not do it on cost grounds. Some buyers want to build to a price after all.

 

The daft thing is, there are a couple of builders who are shifting 25 plus boats a year, are paying more in post sale repairs, reblacking, two pack compensation hull coverings and and repaints than if they  just invested and did the job correctly in the first place.

[Tom Richmond]

When we had our 50x10 dutch barge plated right round the chines, we found out about this all too late... Would have opted for pre primed steel had we known! The yard we were in wouldn't allow blasting, so we took it off mechanically. Took about a week with 2 or 3 people going at it most days, but we felt it necessary to feel confident in the paint finish. 

The new wheelhouse roof which went on at the same time has been rusting away for the last year, when I clean that off with a grinder it comes straight back to bare steel, no scale, with much less effort. As I understand it, many reputable builders let boats sit outside and gather rust for this reason. Obviously they then blast rather than grinding, but it's easier to get back to clean steel regardless. 

 

[Brian422]

I had a boat built in 2009 by a well respected builder and the paint finish was excellent; like it had been sprayed. However, it wasn't until 18 months later that I notice small pin pick sized bubbles erupting on cabin side and front. Boat builder examined boat and said he didn't know what had caused problem but it would require repaint. It wasn't until paint had been on cabin sides had been stripped when painter discovered Mill Scale. Although he worked, on a contracting basis for boat builder, he'd not painted boat from new and didn't know what primer had been used, he did mention that it must be good as it prevented bubbles for some time.

Unfortunately, repaint didn't replicate original mirror finish and although shiny, I've never been completely happy.

Respectfully suggest deal with Mill Scale NOW!

Moral: preparation is everything, without preparation, prepare to fail.

[RLWP]

14 hours ago, BEngo said:

Not illegal, unless you are causing a statutory nuisance through noise or poĺution, say.  Could also be a civil problem.

N

There may be an ancient confusion with sand blasting which caused silicosis. Modern materials and wet blasting have changed that

Richard

[bizzard]

You need a sand blasting cabinet to sand blast your boat in. It comprises of a box on legs with two self sealing arm holes in its front to poke your arms through and a window to watch through whilst you manipulate your boat over and around the sand blast nozzle by hand, waring special tough blast proof gloves. 

Edited July 7, 2017 by bizzard

[Boat&Bikes]

11 hours ago, bizzard said:

You need a sand blasting cabinet to sand blast your boat in. It comprises of a box on legs with two self sealing arm holes in its front to poke your arms through and a window to watch through whilst you manipulate your boat over and around the sand blast nozzle by hand, waring special tough blast proof gloves. 

Had a cabinet like this at the company I sold last year. But was a room where the operative worked in a special tough suit...

[Davebolli]

Thanks very much for all your replies, clearly the consensus is that it's best to prep the steel properly by removing all the Mill Scale. (Pretty much as I expected).

My present boat (Dutch Steel Motor Cruiser) has two pack (Awlgrip) throughout, and it gets lifted every 2 or 3  years for Antifoul only. The paint above and below the waterline has never needed any attention in 17 years. 

I am beginning to realise that there is a massive difference tween the high volume yards and the smaller builders in terms of quality, but demand is such that the volume yards dominate and surprisingly even demand high resale values. I guess this comes down to the fact that if it floats, why worry.

[Bazza954]

This is what happens when the prep work is not done correctly