When blasting the inside of a tank floor we all know that the abrasive often starts piling up everywhere and you have to keep moving it out of the way to be able to see what you are blasting.
If the blaster works in smaller areas with no clean up plan, the abrasive mounds get higher and it is harder to push out of the way.
Many blasters have found that if you start in the middle and work your way out, the abrasive is being dispersed over a larger area so it does not mount up and is blown out of the way as the blaster works further out cleaning the floor as he goes.
This means that by the time you have finished blasting the tank most of the abrasive will be towards the inside edge of the tank close to the manholes. Removing the abrasive is now a simpler task as it is not as far to carry it.
It is far more efficient to start in the centre of the tank and work out from the centre towards the wall, blasting the base of the columns on the way through.