Gritblasting is not allowed where I am moored so I had to do the prep by hand. I used a rotary tool in a drill called a Terco Blaster which is supposed to produce a finish like gritblasting. It is very time consuming so I concentrated on the vunerable areas ie. waterline, pitted areas then welding seams. Managed to treat them all but it took about 6 hours all in all (and it's only a 40ft boat).9 months on and there's no rust at the waterline and the blacking is still intact but only time will tell. I'll get the boat out again next year and see how it has performed and treat any areas showing signs of deterioration or not treated the first time.
Not sure if this level of prep is necessary, I don't think the oil rig legs get this much treatment.
Edit - just to add I've had the boat blacked using bitumen 3 times and each time there was clear rust at the waterline in under a year. I'm moored in a marina environment with lots of boats around me.