Sand Blasting

Strategy

Getting metal from the supplier that is already wheelabraded or sand blasted and primed is a good way to avoid a lot of work especially if your neighbors are too close to sand blast in your yard. The suppliers in Tulsa don't normally sell steel for boats so getting the steel already wheelabraded or sand blasted and primed was not an option. The neighbors are great, and as usual we have more time than money.

We can rent a 185 cfm compressor and 300 lb pot for $650 a week (2008) in order to sand blast the steel to white and then prime it.  However having spent all of our cash flow on steel we don't have the money to get primer at retail prices, so we are scrounge for deals on paint and blast and paint as primer becomes available and buy the rest of what we need once the hull is pulled together. Since the retail price of primer coat is $85 a gallon or about 40 cents a square foot per side, we will also save a little money on not painting steel plate that is just going to become scrap.

Equipment

The big 185+ cfm compressors and 300 lb pots will make a few of days work out of the blasting and when most of the hull is together that is what we will do.  However there is a lot of stuff that will not be possible or easy to reach once they are welded in place.  As well there be late modifications and damaged spots that need to be re-blasted.  For those we will use our current 6 cfm @ 120 psi compressor and a pressure pot blaster from Harbor Freight.  If you get a pressure pot that you intend to run lots of sand through, be sure to get one with the pressure sealing filler hatch.  We the pressure is blown off the pot the hatch will simply drop down opening a big hole in the bowl shaped top so a new load of sand can quickly be added.

Sand

A 100 pound sack of sand will blast  roughly 12 square feet while an engine driven compressor on a small rig will will burn close to a gallon of gas.  So you want to use sand that gets the job done according to your fuel cost as well as your time.  Most people recommend using the sand only once through the pot as it is larger dust after the first pass.  With about 12,000 square feet of steel surface on our 70ft boat, that comes to 1000 bags or 50 tons of sand. There are no beaches in Oklahoma, but we have plenty of sand in the rivers.

Using specialty blasting sand like "Black Beauty" is another option.  There is debate about how many times you can reuse Black Beauty.  Some as only twice, which if that is true then at $10 for 100 pounds the cost of  "Black Beauty" the cost would be $10,000.  Others say it can be reused almost indefinitely as it will pick up steel and slag from the steel being blasted.  So we will experiment with Black Beauty and go from there.

I like to avoid shoveling that much sand so what I imagine is starting with the hull half's prior to folding. Hang one side so the bottom is 6 feet off the ground, then drape a tarp from the top of the plate so that it surrounds the side of the plate so it catches the spent sand and funnels it into a barrel in the back of my pickup. I'll work from the roof of the truck and shoot through an opening in the tarp. The pot stay on the ground and once it is empty, I'll fill it back up from a gate in the collection barrel. Then top it off with some new sand; pull the truck and the trap forward a few feet and repeat. The rest of the plate for the bulkheads and deck can be done the same way.  The only apparent problem with this approach is that I have to prime the steel immediately after it is blasted and I don't have the cash to get the primer yet. 

Mill Scale

45 ft plates fresh off the press   Freshly rolled off a coil it looks really clean. . 7 days later and the mill scale has started to rust. You can't see in in the photo but mill scale sparkles bright sunlight.

Mill scale is a thin layer of  iron oxide that forms unevenly on the surface on steel during hot rolling at the mill.  Our steel was then rolled up onto a huge coil,  loaded on a barge and brought to the Port of Catoosa where Cargill loaded onto a machine that unrolled it from the coil and cold rolled it back into flat plates that were cut to our 45 foot requested length.  That cold roll process smashed the mill scale into the surface so that it can not be seen.  Removing it is required because it will sooner or later loosen and flake off exposing the unprotected steel underneath.  The best first step to removing mill scale is to allow the steel time to rust.  As rust forms it will loosen the mill scale from the surface make it easier to sand blast away.

Profile

The "profile" refers to the roughness of the steels surface after it has been blasted.  Most coatings and paints specify that the best profile is 2 to 3 mil's.  A mil is only one thousandth of an inch or 0.0254 millimeters so you'd need a microscope to see the profile but it is noticeably rough to the touch.  Too little profile and the coating does not have enough surface area to cling to, and too much and the spikes in the surface may stick up through the coating and allow rust to get started.  The tick is to use sand that is the right size. The size of ebrasive particles is measured by the size of the screen that the particle will pass through.  A 40 mesh sand, passes though a screen with holes that are 0.422 mm wide, so window screen will do nicely.  A 40 mesh sand is recommended for cutting a 3 mill profile in the steel.

Ultra High Pressure (UHP) Water Blasting

Requires 20K psi of water pressure. http://www.chariotrobotics.com/industrial_applications.htm
http://www.chariotrobotics.com/Technical%20Articles.htm

Steel Shot Blasting - Wheelabrador - Wheelblast

http://www.blastpromfg.com/products.htm 
www.blastrac.com
Components: http://www.prpplus.com/index.php?page=shop.browse&category_id=22&option=com_virtuemart&Itemid=26
http://impactsamericas.com/index.htm