Water jetting

I have an empty 40ft sail boat hull in steel, I have replaced the deck and cockpit and now need to prepare the steel for paint. The hull had a fire and then sat for many years. Some areas are painted, some rusty (but sound) and some new steel with millscale. I was planning to grit blast with crushed glass but the dust is a problem as there are many boats in winter storage. I can rent locally a water jetting machine which I am told is suitable for steel prep (cost €500/day). I have no experience with this. How effective is water jetting on mill scale, rust and paint? How many hours of jetting should I expect?
I am thinking of applying Jotamastic (by airless spray). Any comments?

 

How many PSI will you get at the nozzle?

-Tom

 

I don't know the specs for the machine I can rent, I was told about it by a steel engineer.
What range of pressure should I be looking for?

 

Unless you get an industrial water blaster, which is considerably more powerful then a top of the line Lowe's/Depot pressure washer, you'll just make more rust.

The usual choice and the economical one, is to media blast the hull shell. Yep, it's dusty, but you can tent the hull shell and contain the dust and debris. Sand is the common and inexpensive media, but coke and walnut shells also can be used, which leave a better finish.

To specifically answer your question, you need a minimum of 5,000 PSI (350 kilo/cm2) to remove scale and surface rust. Of course heavily rusted areas will need more pressure or abrasives.

All this said, you can do a really good job with chemicals, preceded and followed with pressure washing. Naturally, most of these chemicals are going to cause your marina to scream, unless you can completely tent the hull and ground, contain all of the goo and rust and dispose of off site. There are also some biodegradable products that you can use. The cost of all of these products will make you scream, so you're back to abrasives and tarps on a Sunday morning when no one is looking.

 

Ya, what he said.

-Tom

 

Thanks for the replies.
In the end I will probably blast with crushed glass as planned, I have been involved in blasting a few hulls, but I am interested to learn about the alternatives hence the thread.

For €500/day machine rental (no labour) I would expect more than a top line pressure washer...

Sand is cheap yes but causes silicosis, it is also illegal as a blast media in Ireland.

What profile does water jetting create? How does this compare with the profile created by media blasting as a key for the coating?
Has anyone experience of maintaining hulls that have been coated with jotamastic following jetting, I read that it can be applied to humid surfaces, but how tolerant is this?

 

have you thought about using dry ice blasting
works great and nothing to clean up

 

No haven't looked in to dry ice. I want to do the work myself, not only for the cost but I think I would do a better job than a contractor. Trawlers being blasted in the yard often have shadow areas behind frames and poor attention to detail. The contractors seem to clean gravestones and patios most of the time.
Nick.

 

you can buy or rent the equipment
http://www.continentalcarbonic.com/dryiceblasting/index.php
http://www.diamonddryiceblasting.com/blastingequipment.php

or you can do soda blasting and it will not rust as fast and all most the same equipment as sand blasting

 

Glass is made from silica (sand), it's kind of hard to see the difference. Use a good air supplied hood, no matter what. Why is sand illegal as a blast medium?

 

I'm not a chemist or an expert... but if you look it up you'll see that grains of sand shatter in to splinters of silica small enough to enter the cell structure of the lungs and cause cancer similar to asbestosis. Glass is fused silica which shatters in a different way in to particles that are too large to enter the cell structure. Sand is illegal for blasting in many places because of the health risk.
Nick.
Still curious about water jetting. Has no one information to share??

 

It's not difficult nor unreasonable, to take precautions when blasting, regardless of media type. Anyone that isn't doing this needs to have their head examined.

 

You can use a slurry blaster. It is a regular sandblaster with a water input at the nozzle.