You can use the spray cleaner while working from a ladder -- although scaffolding is better -- but practice at ground level first; the force of the spray against the house could knock you off a ladder if you're not careful. Some of these machines come with separate containers you can fill with cleaning solutions or anti-mildew solutions. Sprayers are so powerful that ordinarily you probably won't need to use a cleaning solution; if you do, remember to rinse the surface with clean water afterward.
To maintain a wet edge, start near a corner and run the roller up and down the full height of the wall, moving over slightly with each stroke. Move backward where necessary to even out thick spots or runs. Don’t let the roller become nearly dry; reload it often so that it’s always at least half loaded. Keep the open side of the roller frame facing the area that’s already painted. That puts less pressure on the open side of the roller, so you’re less likely to leave paint ridges when doing DIY wall painting.
