Second floor sanding and painting sounds like a job to have done professionally, not by the few people who show up for working bees. Maintain their enthusiasm by encouraging them to work on a low risk, low skill job such as a weeding party to get rid of that patch of ivy in the corner of the property or planting out the new garden bed at the front and wheelbarrowing in some mulch. Make everyone cups of tea and suggest to someone who can’t handle the heavier work that they make a cake to share with the workers when you all have a break.