Grit blasting nozzle pressure

[blackrose]

A local bloke with a grit blaster has told me that his kit will deliver 130 psi at the nozzle. This doesn't sound like a lot of pressure to me, but then I don't know much about it.

 

Does anyone with knowledge of grit blasting know if this will be enough pressure to remove 3 or 4 coats of Rytex? (now 4 years old but still firmly adhered above the waterline). Where the paint has been chipped off in locks, etc, I can see it's about 1.5 - 2mm thick.

 

Thanks

Edited February 16, 2014 by blackrose

[Burgiesburnin]

On topic but a bit abstract if I may.

Apparently, crushed walnut shells (I think walnuts anyway) are far more effective than grit and can work better with a lower delivery pressure. I think that if used correctly then this is kinder also to the metal that the paint is stuck to.

[blackrose]

On topic but a bit abstract if I may.

Apparently, crushed walnut shells (I think walnuts anyway) are far more effective than grit and can work better with a lower delivery pressure. I think that if used correctly then this is kinder also to the metal that the paint is stuck to.

 

Walnut shells? Perhaps you were thinking of skin exfoliants?

 

http://naturalactives.com/walnut-natural-skin-exfoliant/

 

But seriously, why would walnut shells be better than grit for removing paint? I can certainly see walnut shells being a lot more expensive!

[Radiomariner]

At sea, with large heavy fuel oil burning engines, we used to inject about half a kilo of crushed walnut shell into each turbo-blower every day to clean the deposits off the blades. Very effective, very cheap. The shells are an otherwise unwanted side product

Edited February 16, 2014 by Radiomariner

[BEngo]

130 psi at the nozzle is really just a number without knowing the size of the nozzle. If it's a big nozzle ( large diameter) then it's a quite powerful rig that's going to move a lot of blast media, cover big areas easily and generally spread mess all around.

 

There are many blasting media. What you need depends on what you are trying to shift. For rusty boats chill cast iron grit is good, but very messy, copper slag is good; less aggressive and less messy because the media doesn't rust. Walnut shells will be OK on paint, but slow and expensive on rust, because they are not a 'hard' material. Aircraft engine compressors are cleaned with them, much like RM's turbo compressors. Soda is another possibility, slow, but gentle.

 

The best bet is to talk to someone who does boats regularly about your particular circumstances. The standard you want to achieve after blasting is SA 2 1/2 ( ESS AY Two and a half). Then any epoxy I know will be happy, provided the surface is dry. You can use epoxy direct, or put a blast primer on first. Some blast primers will go onto wet surfaces. Your first coat should be on within 4 hours of the surface being blasted. Read the Product Data sheet carefully for application conditions and techniques.

 

DO NOT underestimate the mess that blasting will make, both in your boat and outside. It will take two of you most of a day to mask off and prepare a 60 ft boat. Every hole needs to be stopped off and every surface where you don't want the paint destroyed has to be protected.

 

But it's all worth it. Epoxy is durable.

 

N

[blackrose]

At sea, with large heavy fuel oil burning engines, we used to inject about half a kilo of crushed walnut shell into each turbo-blower every day to clean the deposits off the blades. Very effective, very cheap. The shells are an otherwise unwanted side product

 

Yes, I realise walnut shells are a food by-product, but they are used in the cosmetics and other industries so they have a market price and aren't "otherwise unwanted".

 

130 psi at the nozzle is really just a number without knowing the size of the nozzle. If it's a big nozzle ( large diameter) then it's a quite powerful rig that's going to move a lot of blast media, cover big areas easily and generally spread mess all around.

 

There are many blasting media. What you need depends on what you are trying to shift. For rusty boats chill cast iron grit is good, but very messy, copper slag is good; less aggressive and less messy because the media doesn't rust. Walnut shells will be OK on paint, but slow and expensive on rust, because they are not a 'hard' material. Aircraft engine compressors are cleaned with them, much like RM's turbo compressors. Soda is another possibility, slow, but gentle.

 

The best bet is to talk to someone who does boats regularly about your particular circumstances. The standard you want to achieve after blasting is SA 2 1/2 ( ESS AY Two and a half). Then any epoxy I know will be happy, provided the surface is dry. You can use epoxy direct, or put a blast primer on first. Some blast primers will go onto wet surfaces. Your first coat should be on within 4 hours of the surface being blasted. Read the Product Data sheet carefully for application conditions and techniques.

 

DO NOT underestimate the mess that blasting will make, both in your boat and outside. It will take two of you most of a day to mask off and prepare a 60 ft boat. Every hole needs to be stopped off and every surface where you don't want the paint destroyed has to be protected.

 

But it's all worth it. Epoxy is durable.

 

N

 

Thanks. I've used the epoxy before and read the specs so am very familiar with it and the surface prep required.

 

I saw the nozzle yesterday - from memory it looked like a half inch hole. The guy is a builder so he uses it to clean old brickwork etc with sand, but he says he can use grit too.

Edited February 16, 2014 by blackrose

[Burgiesburnin]

Walnut shells? Perhaps you were thinking of skin exfoliants?

 

http://naturalactives.com/walnut-natural-skin-exfoliant/

 

But seriously, why would walnut shells be better than grit for removing paint? I can certainly see walnut shells being a lot more expensive!

 

No, I wasn't thinking of skin exfoliant at all, why would you think that I was?

The expense wasn't discussed nor was either grit or walnut shells offered as a cost effective solution.

I offered that walnut shells (I believe) would remove the paint yet be kinder to the metal that the paint was stuck to.

[BEngo]

A half-inch hole is a reasonable size. My suggestion would be to use copper slag. It is not ridiculously expensive, cleans up much better than cast iron grit and doesn't leave rusty marks everywhere when you miss a bit. It also won't wear the nozzle as fast as iron grit. Sand is horrible stuff and can give you silicosis because it breaks down into really small particles when it hits steel. The H&S boys really do not like it, for good reason.

 

Where are you thinking of having it done? Very few boatyards and no docks that I know of will put up with the mess.

 

A good tip is to acquire a small cheapy fridge which still works- the local tip may help. - You can put any left-over mixed epoxy in overnight to keep it cold so it will still be OK to apply in the morning.

 

N

[ditchcrawler]

I would have thought 130 psi to be in the right order but the volume will be large so you will need a good sized compressor. We use to use standard diesel driven compressors. On the point of nut shells, we also used them in Gas Turbine engines to clean the blades. I think someone was using them as a non slip additive for paint in place of sand as you could sand it off to repaint.

[blackrose]

No, I wasn't thinking of skin exfoliant at all, why would you think that I was?

The expense wasn't discussed nor was either grit or walnut shells offered as a cost effective solution.

I offered that walnut shells (I believe) would remove the paint yet be kinder to the metal that the paint was stuck to.

 

Dude, get a sense of humour - my skin exfoliant comment was a joke.

 

The expense wasn't discussed by you, but it was by me. It's an open forum and the expense is fairly important to me since I'm paying for the job.

 

As far as I am aware grit is kind enough to the steel. It's actually preferable for the steel to have a degree of abrasion prior to painting in order for the epoxy to key to it properly.

 

A half-inch hole is a reasonable size. My suggestion would be to use copper slag. It is not ridiculously expensive, cleans up much better than cast iron grit and doesn't leave rusty marks everywhere when you miss a bit. It also won't wear the nozzle as fast as iron grit. Sand is horrible stuff and can give you silicosis because it breaks down into really small particles when it hits steel. The H&S boys really do not like it, for good reason.

 

Where are you thinking of having it done? Very few boatyards and no docks that I know of will put up with the mess.

 

A good tip is to acquire a small cheapy fridge which still works- the local tip may help. - You can put any left-over mixed epoxy in overnight to keep it cold so it will still be OK to apply in the morning.

 

N

 

Thanks but what about grit? You haven't mentioned it at all?

 

I'm going to use Harvington Dry dock - you can do whatever you want there. I'll only be mixing as much paint as I can use in a day. I've used a wintergrade version of the epoxy many times in winter and I know it won't be usable the next day even if kept overnight in a fridge.

Edited February 16, 2014 by blackrose

[BEngo]

 

 

 

 

Thanks but what about grit? You haven't mentioned it at all?

 

 

What sort of grit? Copper slag is a grit, chill cast iron grit is grit. There are others- silicon carbide, aluminium oxide, garnet, olivine......

 

When doing a boat IMHO what is wanted is a safe ( so very low silica), reasonably priced, reasonably aggressive (so the job doesn't take for ever) and reasonably clean abrasive which will tackle, paint, rust, millscale and general grot. Almost all abrasives will give you a clean finish on steel, eventually. I like copper slag as the grit, because it ticks these boxes and leaves a good paintable surface, for both 'black' areas and ' shiny' areas. There are thousands of blast media suppliers out there; some at least will be local to Harvington and able to give you and your mate some good advice, particularly since he will be able to describe the rig to them so they can advise on what size media to buy, how much he might need, and what they have in stock that will do the job.

 

N

[blackrose]

What sort of grit? Copper slag is a grit, chill cast iron grit is grit. There are others- silicon carbide, aluminium oxide, garnet, olivine......

 

 

Thanks, I didn't realise copper slag or chill cast iron were types of grit. They sound more like lumps of metal, but I guess they are ores.

[RLWP]

 

Thanks, I didn't realise copper slag or chill cast iron were types of grit. They sound more like lumps of metal, but I guess they are ores.

 

They're waste materials from metal processing

 

 

Richard

[blackrose]

 

They're waste materials from metal processing

 

 

Richard

 

So how is that grit then?

[RLWP]

So how is that grit then?

See, the trouble is you are taking 'grit blasting' too literally.

 

There is a huge variety of blast media, from very hard materials down to bicarbonate of soda. Different materials do different jobs

 

'Grit blasting' is what you ask for, then an expert turns up and uses the appropriate media. You'll notice that the phrase 'grit blasting' has replaced 'sand blasting' now that no one does this any more because of silicosis

 

Anyone know if boat hulls are blasted wet or dry? I would have expected wet

 

Richard

 

No, I'm not an expert, just a little more informed.

 

MORE: have a bit of a browse here Mike: http://www.guyson.co.uk/aftersales/blast-abrasives/

Edited February 19, 2014 by RLWP

[ditchcrawler]

See, the trouble is you are taking 'grit blasting' too literally.

 

There is a huge variety of blast media, from very hard materials down to bicarbonate of soda. Different materials do different jobs

 

'Grit blasting' is what you ask for, then an expert turns up and uses the appropriate media. You'll notice that the phrase 'grit blasting' has replaced 'sand blasting' now that no one does this any more because of silicosis

 

Anyone know if boat hulls are blasted wet or dry? I would have expected wet

 

Richard

 

No, I'm not an expert, just a little more informed.

 

MORE: have a bit of a browse here Mike: http://www.guyson.co.uk/aftersales/blast-abrasives/

Both

[RLWP]

Both

Well, that's clear, I was expecting a little more...

 

Richard

[ditchcrawler]

Well, that's clear, I was expecting a little more...

 

Richard

OK sorry, There was company at Crick show a few years ago demonstrating dust free wet blast http://www.quillfalcon.com/ but I think most companies do dry blast.

Edited February 19, 2014 by ditchcrawler

[BEngo]

Wet blasting with media makes less dust, but more spray so may be initially attractive. However, wet blasting is more expensive, because you not only have to have a suitable blast medium for wet work (chill cast iron grit won't work , copper slag won't either), but you also have to pay for a rust inhibitor in the 'water' and a more complicated 'pot and nozzle' set up is needed.

 

Really high pressure (400 bar and on) water blasting uses no media, just inhibitors in the 'water'. The kit is phenomenally expensive, the protective gear needed is nearly as dear and in the hands of an amateur or a cowboy it's lethal. Very effective though.

 

The other factor is that when your boat has been wet blasted a wet-substrate primer is needed, because if you wait till it dries out properly it will usually have gone rusty again (in UK) so blasting it was a waste of time/money. Dry blasting allows you to put the first coat of epoxy pitch direct onto the blasted surface. Again this saves money because you only need the top two coats.

 

N

[blackrose]

Well like many of these subjects, the apparently simple job of taking old paint off a boat hull is obviously a lot more complex and involved than the layman would have imagined.

Edited February 20, 2014 by blackrose

[RLWP]

Well like many of these subjects, the apparently simple job of taking old paint off a boat hull is obviously a lot more complex and involved than the layman would have imagined.

 

Interesting though. It would be a poor day in which I didn't learn something

 

Richard

[Ex- Member]

Well like many of these subjects, the apparently simple job of taking old paint off a boat hull is obviously a lot more complex and involved than the layman would have imagined.

 

It's not complex at all.

 

If you tell the grit/shot/sand blaster the material he's required to blast ie (steel) then he'll turn up with the appropriate blast medium. The blaster who turned up to do ours had several different grates in his truck, I told him the application was a 2 pack paint and he simply blasted with the required grade.

[David]

Hi

I see one poster has mention "the volume will be large so you will need a good sized compressor", having had access to all the equipment via a grit blast equipment manufacturers near Chester who hired out kits and sold grits and a compressors via my brother who was sufficiently high up in H++++n Hire to "lose a compressor" for a week or two I have blasted our old boats 6 times in 30 years of boating, admitted the last time was about 1994 anyway the compressor I used was usually about a 600 cfm (cubic foot per min) with a 3/8" or 7/16" nozzle and 1/8" slag grit. Cost in 1994 £120 for the kit and protective helmet with air feed and 10 mask lenses, £5 per half cwt bag of grit (needed between 15 and 20 bags) nil for compressor but would have been about £300 per week and also nil for diesel (45 gallon drum came with comp') but it used most of that. Below are some figures from a site I have linked, the second table indicates why proper equipment is a must for safety, scary speeds will remove flesh to the bone and eyes gone instantly.

 

Production Compressor size required nozzle rate at 100 psi rate

 

nozzle size compressor size

1/4" 185 cfm 40-50 h.p.

5/16" 250 cfm 60-75 h.p.

3/8" 375 cfm 75-100 h.p.

7/16" 450 cfm 125 h.p.

1/2" 600 cfm 150 h.p.

 

Blast Nozzle Pressure Estimated Abrasive Velocity

140 psi 588 mph

125 psi 525 mph

110 psi 462 mph

100 psi 420 mph

95 psi 400 mph

90 psi 365 mph

85 psi 330 mph

80 psi 270 mph

75 psi 210 mph

70 psi 190 mph

 

Link http://www.dawson-macdonald.com/media/parts-and-accessories/BorideNozzleCatalog.pdf

[blackrose]

 

It's not complex at all.

 

If you tell the grit/shot/sand blaster the material he's required to blast ie (steel) then he'll turn up with the appropriate blast medium. The blaster who turned up to do ours had several different grates in his truck, I told him the application was a 2 pack paint and he simply blasted with the required grade.

 

If it was only that simple. After meeting up with one local grit blasting operative, I've got the feeling that they're not all necessarily as professional and well-equipped as the blasting company that did your boat. I've now spoken to a few of them and It's not just the material being blasted that they want to know about - it's the existing paint that's on there too. And they're not all sure which grit would be best to get 3 or 4 coats of blacking and two coats of Primocon off - or even if their equipment is up to it!

Edited February 24, 2014 by blackrose

[RLWP]

Mike, that's quite reassuring. I'd be happier with a company that wanted to have a think about what they are doing

 

Richard