Interesting proposal by Rei-sensei, I believe she did it both because hands are very expressive with all the different nuances they convey with the great range of motion and variety of poses they have, and because it justifies the small canvas that would be a little gentler on the need for supplies, and also because it is something completely different from what she saw he painted in the room at the Amami household, so it'll maybe be a new obstacle and reality check for him, while also having the potential to be a new spark of curiosity for him.
I can't say anything about painting hands, but drawing them, that can be both a hellish pain in the ass but also extremely fun and interesting. It's no wonder Kohei Horikoshi loves to draw them and uses one as his icon on shonen jump.
Also, I wonder if he'll with a clenched hand, showing his frustration, or maybe the hand bashing a invisible table (I doubt it, he's too nice), or go for something unexpected like the hand spilling out of the canvas as if trying to grab something outside. I'm really curious to what each of them cooked up.
Again, thank you very much for the scanlation. ♡(ŐωŐ人)