Dry Ice or Sandblast

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swanson37

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Howdy Folks,

Further to a post I made a while back about some DIY osmosis treatment I was planning I have bitten the (very expensive) bullet and finally decided to get the hull blasted in some way to open up the blisters and get the boat back to the gel coat.

I know for a fact that she has not been taken back to the gel coat for a number of years (I have owned her for coming up on five years and the previous owner had her for at least three and did no maintenance of any description). In that time she has been antifouled every couple of years and the only time the antifoul will have been taken off will have been during her monthly scrub by a diver (our boats stay in the water year round) which won't have removed a lot of paint. So she is going to have a lot of coats of paint on her.

I was initially going for dry ice, beacuse it is supposed to be a bit more gentle on the gel coat, but I'm hearing that the dry ice route may take a lot longer than initially suggested and what with the many layers of paint I'd be better going for a sandblast..particularly since I am trying to burst open some blisters at the same time...

So what does the esteemed panel think? I'm leaning towards the sandblasting route as I think it may be more effective....

Answers on an (electronic) postcard please

Cheers
 
As you discovered dry ice is slow and expensive, primarily used for cleaning electrical devices or to avoid moisture contamination.

However, Sand blasting is the almost too aggressive for relatively soft resins and gel coats.

Have you looked at slurry blasting, no dust and softer.

If your blisters are only small a simple wet sand with an air driven belt sander will remove the antifoul and expose the blisters, then a grinder will allow you chase the staining and remove all traces of the problem. Preparation is the is the most expensive part of treating a hull.

If you are not confident you can do the actual repair and fairing, then use a professional to the this latter stage, though it's not rocket science.

I hope this helps.

Avagoodweekend......
 
Dry Ice or Sand Blasting

As you've found dry ice is slow and too gentle for what you want. Can't speak for price in Oz but certainly expensive in Europe.
Sand blasting is a generic term and sand should never be the medium for both efficiency and safety reasons. Slurry is probably best for what you want. It's quick and aggressive enough while being very controllable (in the right hands) and it keeps mess down to a low level.

Slurry will open some of the blisters but you will probably have to go over the hull carefully to find others. If you want the blaster to be more aggressive and open everything up, which may be worthwhile if the blistering is extensive, you must expect a lot of the gelcoat to be removed, so you will need to plan on rebuilding the waterproof membrane with epoxy - solvent free for at least the first 2 coats.

Interesting comment by Oldsaltoz about the cost of blasting. Over here it would be less than 20% of the cost of osmosis treatment!
 
Hurley24 said:
Interesting comment by Oldsaltoz about the cost of blasting. Over here it would be less than 20% of the cost of osmosis treatment!
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The cost I referred to was the "preparation" not just the blasting.

This includes all cleaning, sanding, grinding, blasting, more cleaning and all labour and materials, everything but for the application of a suitable protective coating and any fibreglassing.

Avagoodweekend......
 
oldsaltoz said:
The cost I referred to was the "preparation" not just the blasting.

This includes all cleaning, sanding, grinding, blasting, more cleaning and all labour and materials, everything but for the application of a suitable protective coating and any fibreglassing.

Avagoodweekend......
Click to expand...

That's what comes of pretending to be on west coast time when your body still feels its in the UK. Next time I'll try to read the post properly!
 
The chap that grit blasted my hull had the skill to take off a lot or a little material, depending on the state he found the hull was in. So I would go for the man / person who has done this task before and therefore has the skill and equipment to do your job. Ask about.

73s de
Johnth
 
In amongst costs, you've got to include drying time, which if you're in a commercial yard will cost you.

Incidentally, had mine slurry blasted (not for osmosis, just for a decade or more of dead antifoul) - quick, cheap (relatively) and left a great surface for primer.
 
After buying my boat a year ago I had many years of a/foul dry blasted off the hull. It exposed that the boat had previously been stripped and epoxied (15 years earlier I subsequently discovered) which had been quite invisible.
The blasting did a superb job, it left a perfect surface for the a/foul primer (Jotun Vinyguard), and blew out a few fingernail size patches of epoxy that had never adhered properly, which I epoxy puttied.
The rudders had not been epoxied, it left the gel coat of those beautifully prepared for priming too.
I think it's in the skill of the operator. The guy who did ours (remarkably cheaply too) has been doing it for some 20 years, nothing but boats. (It was Tony from Brighton Marina, for anyone wanting a UK south coast reference).
He blasted with a substance called Olivine which I think is a powdered volcanic rock but I'm not sure. He said that using sand is now illegal.
If ever I need to I will happily go for dry blasting again.
 
Fantasie 19 said:
The chaps at my club are thinking about clubbing together to get a bloke down who uses the farrow system of blasting - might be another option...??

No connection, but it certainly sounded interesting...

http://www.farrowsystem.com/
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If you manage to pull this off see if you can also talk them into hiring an airless spray kit to apply the new antifoul, takes minutes to complete and leaves a very nice smooth finish, much better than any roller. We did this when I was still sailing mono's.

Avagoodweekend......
 
I haven't read most of the thread, I did read something that implied that sand was too harsh. That I can say for certain is untrue, I've actually done a few, with sand without any problem at all. The finish is excellent without any damage to the hull at all, I can honestly say that there is little skill to it.

The main merit of dry ice or soda is the lack of residue, dry ice I believe to be much slower than sand, and more expensive. The main issue with dry sand is the stuff blowing everywhere and the residue of course other than that it's fine. A true wet slurry blaster will reduce the dispersion, but not completely and the problem that then arises are the pools of poisonous sand and water. The used sand slurry is awkward to deal with. I'm not convinced the Farrow system is greatly different to wet sand.

Best equipment is ultra high pressure water (30,000 + psi) which is extremely expensive, heavy and dangerous and does require training, skill and experience.
 
Yes, I agree, sand makes a good job and makes a mess too. Most other blast systems make a mess and make a good job as well.

It is a skill to make a good blasting job though. It is possible to drill a hole or take the shell from an egg, if you have the skill to do it with a blast system.

Because of the mess, A lot of yards will not let a blasting process on or near their site. I think sand is black balled in the UK because of the risk of sylicosis (spelling ?)

73s de
Johnth
 
Had a boat done with the Farrow system a couple of years ago and it was excellent. The end product was immaculately clean with little gel removal.

As far as I can gather, the issue with any blasting is the balance between the time they take and the aggressiveness of the removal. If they do a quick cheap job, they have to turn the pressure up and go for it. Inevitably you will get areas with more taken off than others. So in other words its not so much the medium they use as the pressure they use and the time they take.

For example, the Farrow job took a day and a half on a 34 footer. Hopelessly uneconomic for the guy doing the work ( it was near enough his first job) but a great result. The same equipment and media can be used to etch or cut glass so going through GRP would be no problem.
 
Can't agree about the skill level required. In my opinion, and I've successfully blasted quite a few, if you can hose your garden path down you can blast a boat.
Funnily enough it's easier to evenly blast with bigger and more powerful equipment than with small. I had a 300 and off cfm compressor and slurry blast unit that would enable me to stand about 6ft away. It would take about an hour to do a 30ft boat. The setting up cleaning etc would take far longer.
 
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