There are two kinds of body filler: one-part, and two-part.
The two-part is typically catalyzed polyester filler. Smells like hell’s dumpster, cures pretty fast. I’ve had good results with Evercoat Rage Optex - it changes color from pink to green as it cures. Easy to sand. Some fillers can become pretty difficult to sand if left overnight (Bondo brand comes to mind). You get what you pay for, and I’d rather work hard at my “real job” and sweat less for my hobby.
The other type is a one-part putty: toluene evaporates, and it dries. It’s best for very thin layers, and is best if left to dry overnight before sanding.
I also used Clean Sheetsfor a mixing board.
https://smile.amazon.com/dp/B00AZA1XVK/ref=cm_sw_r_cp_api_i_FzuyFbMK5RK25 It came with a spreader that has a rounded edge that was pretty perfect for my 1” radius fillets. It’s also a good size because you don’t want to mix too much at once.
I’ll also note that Dura-block makes a 1” round sanding block. It was ok, but slow. I ended up using a Harbor Freight 2” random orbital sander (compressed air powered), and 180 grit discs. It seems kind of counter intuitive, but it actually made the joints beautifully smooth and rounded. It did, however, use air like crazy...