If building in steel, you come to the point of needing to prepare the hull and fittings for some manner of coating. Often times this means sandblasting. By all accounts a miserable job.
You can buy sandblasting equipment, rent it, or, as Tom points out in a recent Origami Boats thread, build it.
EDIT: Previously you could find some pictures as referenced in the next paragraph, but it seems to be gone now, sorry. Check the other link lower down.
You can find some pictures of his rig (and a very nice looking craft he’s using it on) if you are a member of the group at:
http://ph.groups.yahoo.com/group/origamiboats2/photos/view/85f9?b=63&m=f&o=0 (there are several pictures, check them all).
There are obvious cautions to this type of construction… there are pressures involved that could cause injury or death, so please make sure you know what you are doing and that the vessel (pressure pot) you use is able to safely handle it.
You can also find instructions about the desing and building of a pressure pot sandblaster at Vintage Projects which has a lot of other interesting ideas.
Another take on the them was provided by Sam:
I built one using a 100# propane tank, one of those about 1′ in diameter x 4′ tall. At the time I was working for someone blasting bulk oil tanks at tank farms, so I copied the features of the pot we used with the 600 cubic foot compressor which had a 1" blast nozzle. My compressor was a 60 c.f. The pot was actually pressurized to operating pressure of 100lbs. and also used gravity and siphon to feed sand to the airstream. I used standard pipe and fittings for everything including the blast nozzle, which I think was about 3/16 or maybe 1/4". The hose was some sort of 3/4" rubber hose from a farm supply place that had enough pressure rating. The nozzle was a 6" piece of 3/4" pipe that reduced down to the 3/16" size, the 3/16" piece was occasionally replaced as it wore out from the sand. The hose generally wore out right near the nozzle, cutting a few feet off once in awhile remedied that. I did about 10 houseboats, a house, a barn, some cars, 25 1000 gallon farm tanks, a 10’dia. x 50′ ammonia tank and 5 12′ dia. x 20′ high oil tanks and I think I only replaced the hose once. When it was adjusted right it used very little sand compared to what I see some people use down at the local boatyard. Sam