Shot Blasting New Steel ?

[Lou - Rowanberry]

Hi there,

 

Does anyone know how important it is that new steel is shot blasted before 2-pack paint is applied?

 

We are looking in to a new-build and have been informed of the potential for mill-scale to cause bubbles as it lifts away from the steel (as steel expands/contracts) thus lifting away the paint too.

 

Any informed opinions on this much appreciated.

 

Many thanks,

 

Louise

[jake_crew]

As you say, new steel will have mill scale detritus on the surface, as well as process oil etc.

 

Unless it was specified as bright, which means it was etched in acid to clean it.

 

None the less, there will be oil from the rolling process which also keeps it rust free.

 

Welds often have a "glassy" surface which paint doesn't stick to well, so shot / sand blasting make sense to give the best paint adhesion.

[blackrose]

When my boat was recently grit blasted some of the mill scale still wouldn't come off!

[nicknorman]

Our new build boat wasn't shot blasted (actually folk grit blast these days, rather than using lead shot!) and the initial blacking came off fairly quickly, some in chunks. It obviously wasn't bonded very well. We then had the hull blasted and 2-packed, 3 years later it's still great. If I were having another boat built I'd want it blasted, and even more so if I were going to have 2-pack put on.

[nb Innisfree]

After the first few months afloat Innisfree's blacking peeled off from the millscale, this despite a thorough decreasing before blacking two years earlier (it was out of the water during that period) We had the boat craned out soon after but grit blasting failed to remove all the millscale, we should have waited longer but being a newbie the rust developing underwater worried me.

 

During fit out on dry land I would recommend letting the hull rust untill millscale had gone then grit or shot blast immediately before coating.

[Batavia]

For best results, the grit blasting should meet the specification recommended by the coating supplier - typically this would be Sa 2 1/2, to ISO 8501-1. This requires the removal of all mill scale and other foreign matter and leaves a surface which feels unpleasantly rough.

 

Blackrose's results (above) might have met Sa 2 - and were probably quite satisfactory for most practical purposes.

 

Chris G

[DHutch]

Our boat was grit blasted back to bright steel when new, inside and out, and painted with good quality epoxy blast primer, followed by twopack polyurethane on the inside and on the cabin, and a twopack epoxy blacking. We have now switched to a onepack blacking of similar cost/quality which we also find to be very good.

 

The decision to do this was before my time, but if I where to spec a boat again I would certainly ensure that mill scale was removed along with the oxide on the weld surface, before painting. The only areas we have any rush issues with at all on the boat are areas such as the underside of the handrails and other returns where the blasting was not able to reach fully.

 

Good paint and the right prep means you can do four years between blacking not two and cabin paint that lasts 8-10years, and have effectually zero paint loss and or rust between repaints.

 

 

 

Daniel

[DHutch]

I might start a dedicated thread, but as ultimate aim of surface prep and good paint is how long it lasts I have attached some photos below of the boat taken during our week on drydock in September.

 

Due to various reasons this time it had been five years since the last docking rather than the normal four which was noted by slightly worse condition of the paint. However given the boat has been in the water the time, and is used extensively through each year by a very mixed crew, I think you would say the pain is in good condition. The cabin paint at this point is nine years old.

 

As the photos where/as taken largely for our personal reference if anything they are of the worse areas rather than the better areas. As you will see, of what little paint loss there is, almost all of it is on the soft-chine and other areas which stick out a little, due to abrasion damage.

 

We revise our technique occasionally depending what we find and new advice, but basically each docking the hull is simply pressure washed off, loose areas are now attacked with a needle scaler, and then all remaining paint lightly buffed/keyed with a rotary wire brush which also removes the last of any stubborn algy etc.

 

Going onto dock and hull condition

 

Hull condition including close up.

 

Epoxy ali primer, followed by two coats of blacking.

 

 

Daniel

[peterboat]

when my boat was stretched the extension was left to go rusty and then ground clean when it was shot blasted clean for zingering this new section cleaned up a lot easier than the old section. i was building a new boat i would have it shot blasted and zingered then zinger blacked rather than 2 pack whilst it is more expensive you do get what you pay for and the finish is very hard to say the least.

[dave moore]

Most top end builders will grit blast as a matter of course, assuming that they have somewhere suitable to do the job. The process is made easier if new steelwork is allowed to rust up first, making the mill scale more easily removable. The abraded steel has a textured feel to it and is an ideal ground for suitable primers. If possible, I'd advocate it for every new build. Some of the yards I work with also use it as a way of removing old paint and mill scale prior to repainting, though the sealing up has to be meticulous...the dust gets everywhere! Hope this helps.

 

Dave

[Giant]

Another possibility is to have the steel supplied to the yard already shot blasted and epoxy primed.

 

When we had our overplating done this was in our surveyor's specification for the work. Once the welding was finished the seams just needed a bit of prep and then the whole lot could be overcoated.

[raymondh]

Another possibility is to have the steel supplied to the yard already shot blasted and epoxy primed.

 

When we had our overplating done this was in our surveyor's specification for the work. Once the welding was finished the seams just needed a bit of prep and then the whole lot could be overcoated.

All the main steel used on our boat was supplied blasted and epoxy primed (see my gallery for pics) it depends on what the builder is prepared to pay

Ray

[DHutch]

Another possibility is to have the steel supplied to the yard already shot blasted and epoxy primed.

 

...Once the welding was finished the seams just needed a bit of prep and then the whole lot could be overcoated.

 

Which is fine, as long as the welding is preped suitable, if not, it will rust.

 

- One of the best ways to prep the weld is to the shot/grit blast it.....

Our new build boat wasn't shot blasted (actually folk grit blast these days, rather than using lead shot!) ....

I cant speak for all mobile outfits but at work they where certainly using metallic (steel I thought) shot in the shotblast process.

 

 

Daniel

[Mike Jordan]

Like many things in boat building it can be done well and be a great basis for a lasting finish or very sketchily to get an appearance that looks similar. As a DIY task grit blasting must be the worst job in the world! Everything gets covered in a black snow of grit and mill scale which all needs to be cleaned up to allow you to see the workpiece to check it. This is particularly grim inside the shell but quite easy on the outer surfaces. I only did it once and it's a very memorable job. This is probably the reason that some yards will settle for a swift peppering of the shell with grit to get an even texture rather than a proper job.

On the last occasion I built a shell a specialist contractor blasted it it what he referred to as Swedish white, I don't know what this refers to but it meant a really good job to my eye. To do the job well the paint needs to be applied immediately to ensure that no moisture gets near the steel, even putting a sweaty hand on it should be avoided.

If you are paying for it, inspect it before the paint goes on, is my suggestion.

Mike.

[peterboat]

you could file your nails on mine after it was done very rough which is what zinger needs

[ditchcrawler]

Like many things in boat building it can be done well and be a great basis for a lasting finish or very sketchily to get an appearance that looks similar. As a DIY task grit blasting must be the worst job in the world! Everything gets covered in a black snow of grit and mill scale which all needs to be cleaned up to allow you to see the workpiece to check it. This is particularly grim inside the shell but quite easy on the outer surfaces. I only did it once and it's a very memorable job. This is probably the reason that some yards will settle for a swift peppering of the shell with grit to get an even texture rather than a proper job.

On the last occasion I built a shell a specialist contractor blasted it it what he referred to as Swedish white, I don't know what this refers to but it meant a really good job to my eye. To do the job well the paint needs to be applied immediately to ensure that no moisture gets near the steel, even putting a sweaty hand on it should be avoided.

If you are paying for it, inspect it before the paint goes on, is my suggestion.

Mike.

Spot on. Prime as soon as the blaster has finished.

[Batavia]

On the last occasion I built a shell a specialist contractor blasted it it what he referred to as Swedish white, I don't know what this refers to but it meant a really good job to my eye.

The Sa numbers that are used to define the surface preparation standards were originally from a Swedish standard (now incorporated in ISO 8501-1). Sa 2 1/2 and Sa 3 have definite whiteish appearance after blasting - hence possibly your blaster's reference to "Swedish White" (unless he was referring to his girlfriend).

 

If the job is being done thoroughly, the painting inspector will also measure the surface roughness.

 

Chris G