Shot Blasting - Dumb newbie question

[KeelsonGraham]

Hi all,

I’m new to boat ownership and have recently opted to have the anti foul shot blasted off so that I can start afresh with a new base layer. I assumed this would just take it down to the gel goat, but now I’m not so sure. Does shot blasting also remove the gelcoat?

 

[penfold]

More likely grit(or even soda or dry ice) blasting; shot blasting is really only for cast iron keels or steel hulls. If you've asked for anti-foul removal I'd expect just the antifoul removed; any contractor who takes the gelcoat off when it's not been asked for won't be in business long.

 

[Paulfireblade]

I am sure someone far more knowledgeable will be along soon but no blasting for removal of anti foul won’t remove gelcoat as that would be catastrophic but it can leave tiny indentations in the gelcoat if there are air bubbles near the surface

 

[Sandy]

Talk it over with the people who you employ to do the job. They will do exactly what you request.

 

[savageseadog]

I've actually blasted a few boats. It's actually quite easy to sand/grit blast without damaging the gelcoat. Wet/slurry blasting reduces dust and makes the job even easier. If there are voids in the laminate or osmotic blisters, blasting may blast through the gelcoat but that's a good thing

 

[Tranona]

Depends on what you ask the contractor to do. "shot blasting" covers a whole range of treatments, techniques and blasting media, As suggested it is quite possible to just remove the old antifouling without damaging the gel coat. If you are doing this then it is worth considering using Coppercoat rather than epoxy and conventional antifoul and the extra cost is relatively small. Most of the cost is in the preparation rather than the final coating.

 

[Gsailor]

No such thing as a dumb question from someone new to a subject.

 

[penfold]

Shot blasting is a very specific thing; it's blasting things with steel shot, typically driven with compressed air. Don't ask for it as it will remove gelcoat and probably laminate as well.

 

[arcot]

Shot blasting is a very specific thing; it's blasting things with steel shot, typically driven with compressed air. Don't ask for it as it will remove gelcoat and probably laminate as well.

In many jurisdictions blasting with sand is prohibited.
Lung disease
Black grit made from nickel slag
is used instead

 

[savageseadog]

In many jurisdictions blasting with sand is prohibited.
Lung disease
Black grit made from nickel slag
is used instead

"Sand" is a generic term used to describe various media used in the process, normal silica sand shouldn't be used. The black stuff is copper refining slag, iron silicate. "Shot Blast" can be used generically as well although strictly it's a blast process that uses steel shot that's inappropriate for GRP boat blasting. The steel shot is normally used in an enclosed recycling plant anyway.

 

[Neeves]

"Sand" is a generic term used to describe various media used in the process, normal silica sand shouldn't be used. The black stuff is copper refining slag, iron silicate. "Shot Blast" can be used generically as well although strictly it's a blast process that uses steel shot that's inappropriate for GRP boat blasting. The steel shot is normally used in an enclosed recycling plant anyway.

Interesting.

I'm not aware that there is much copper or nickel melting in the UK so the amount of slag will be insignificant. It could be imported crushed and graded - but I would have thought that, decimated though the industry might be, Fe blast furnace or steel slag would be more abundant and have a not dissimilar composition to any other metallurgical slag (mostly mixed, Mg, Fe, Al, Ca silicates),

Jonathan

 

[arcot]

Grit blasting is the generic description

 

[savageseadog]

Interesting.

I'm not aware that there is much copper or nickel melting in the UK so the amount of slag will be insignificant. It could be imported crushed and graded - but I would have thought that, decimated though the industry might be, Fe blast furnace or steel slag would be more abundant and have a not dissimilar composition to any other metallurgical slag (mostly mixed, Mg, Fe, Al, Ca silicates),

Jonathan

I think it is imported.

"Grit" "Sand" "Shot" are all used generically and interchangeably by people. "Shot Blasting" is an obviously different process of course.

 

[Neeves]

I think it is imported.

"Grit" "Sand" "Shot" are all used generically and interchangeably by people. "Shot Blasting" is an obviously different process of course.

Decidedly thread drift

One of the issues with production of metal from ore was

What to do with the slag.

It found tonnage application as road stone, ballast for railway lines. Slag is 'glass like' and if crushed and graded would have commercial value as an abrasive. I recall in the mid '80s garnet was introduced (from South Africa?) and was exceptionally hard, difficult to crush. I note that abrasive papers are now using a red abrasive - which could be garnet.

Jonathan

 

[savageseadog]

Garnet is definitely sold as a blast media, it's significantly more expensive than the copper slag based J-Blast and smaller grit size. It's used for higher quality work.

A little local mystery. In a field adjacent to our property I found a number of green glassy rocks which I took to the finds curator in Chester Museum along with some pottery. She suggested the rocks were slag. I don't know if the stuff was brought to this rural spot to perhaps create a path, it seems improbable that there was industry round here., I know there were some dwellings long demolished.

 

[Neeves]

Garnet is definitely sold as a blast media, it's significantly more expensive than the copper slag based J-Blast and smaller grit size. It's used for higher quality work.

A little local mystery. In a field adjacent to our property I found a number of green glassy rocks which I took to the finds curator in Chester Museum along with some pottery. She suggested the rocks were slag. I don't know if the stuff was brought to this rural spot to perhaps create a path, it seems improbable that there was industry round here., I know there were some dwellings long demolished.

You are taxing my memory.

Glassy rocks, obsidian, flint (or slag). Historically metal smelting was small scale, slag became an issue as furnaces grew bigger. Anomalies are common place - ballast for ships was commonly rock, or boulders. When the ballast was not needed, as the cargo was the ballast the rocks/boulders were dumped. Some boulders had commercial value ( in areas that was devoid of boulders or devoid of hard rock) etc etc. Glassy rock could also be contaminated glass - but this would commonly have been remelted.

I saw some of the garnet feed stock (around 1985), they were 30mm, (I think) hexagonal crystals - they were so hard to be unusable (then).

Interesting - but not of much value to the OP

Jonathan

 

[Stemar]

I've no idea of the cost compared with other materials, but dry ice would be my choice, as that only leaves the antifoul to clear up, and no abrasive grit

 

[johnalison]

The boat may have a coat of epoxy over the gelcoat. I had mine done some years ago and was very happy with the result. After the blasting the surface was mostly a matt finish with some tiny voids that the folk who did the next stage filled in minute detail before adding more epoxy and a/f.

 

[zoidberg]

It is practicable to use 'soda crystals' for the removal of old antifoul coatings. This material is far less aggressive than many others used. There is a specialist company who use 'dry ice'....

I used 'soda crystals' and did my job myself - as South Coast companies wouldn't travel to me. It eventually turned out successful - and cheap.

Lack of knowledge/lack of skill/incompetence can result in A: abrasion damage to hull laminate, and B: abrasion damage to operator. It is a matter of conjecture which is the more important....

 

[UK-WOOZY]

used Falcon blast from essex to do my hull sandblasting and it just went to the gelcoat. Very happy with the results back in 2020.

Sand blasting | Falcon Blast Ltd