The Sumo chassis is available in bare steel or galvanised, the vast majority of builders choose the galvanised option as the extra cost is not high and it saves huge amounts of work in preparing and painting the chassis and provided long lasting protection that is superior to any paint job or powder coat.
Drilling out holes
If you choose the galvanised option, many of the holes that are pre-drilled will become coated in zinc plating and so need to be drilled out before work commences. It is far easier to spend an hour going over the chassis and drilling out these holes before you start work assembling as access will become a problem as the car is built.
Sealing the Chassis
There are a few areas of the chassis that if left may let water in, or catch water and be a potential place of corrosion. The advice is to use a polyurethane sealant such as the Sikaflex that Pilgrim can supply to seal these areas. These are around the floor pan, boot edges etc.
An alternative polyurethane sealant/adhesive to Sikaflex is Polyflex-LM. This is available from Decorating Direct, as well as other suppliers, and is much cheaper.
Here is a painted rolling chasis.