Concrete pool no main drain so how do you paint it?

If there is no drain at the bottom or connected to the pump unit then you will have to get or rent a pump. You can usually rent one at Home Depot or a rental place. Some of these pump can several hundred gallons an hour so it would be faster than trying to do this with the old siphoning method. Depending on the area of the country you live you dont want the water out of the pool for very long cause a low water table can actually push the concrete up without the weight of the water in it. This is the way we did it when I used to have a concrete pool and would have to have the liner fixed or replaced. I used a submersible pump so that it got down really close to the bottom on the pool. Good luck.