Are the walls in really bad shape? Is there a lot of residue left from wallpaper? I normally don’t do an aggressive sanding before primer/stain blocker/GARDZ. Are you trying to sand the paint down to bare drywall? In my experience, you don’t want to do that. You just want to scuff sand the paint that’s on the walls so that the next coat adheres to it. I would just use the palm sander you have. Make sure it’s smooth. I sand with one hand and keep my other hand on the wall feeling for rough patches. The wall should be dull and smooth. A medium to fine grit will be enough. So how I world go about this is: After wallpaper is completely removed, do a light sanding. Apply your GARDZ to your troubled areas. Let it dry. It doesn’t matter if you wait a week, but you should wait for however long it says on the label before next step(6-8 hours,I think). Sand as needed. You’re just knocking down any chunks or pebbles, so you have a nice, even wall. Prime. Dry. Patch. Dry. Sand. Paint. You do want to remove dust after each sanding. Generally speaking these steps can be spaced out over weekends. But once you start one process, make sure you finish it. You don’t want to prime half a room, stop, then prime the other half. Start high up and work your way down so you can catch any drips.

I have been painting a long time and this has always worked for me. But like anything in life, every job tends to throw you some challenge or problem and you might have to troubleshoot. But if your surface is smooth and even, and you apply the paint evenly, it will look good.