[Zeke]

If you have a belt sander and a time constraint, I don't think you have much choice. Try to use more than just your arms and elbows if it's low on the wall. Go through the grits. Don't spend a lot of time with heavy grit, just get things going in the right direction. I assume you will have to start cross grain/cross boards. A RO sander starting with 100 should get the wood going back towards being paintable. Nothing past 180 is needed for primer.