Outside- clear dish soap and a kitchen scrubbing pad, and steel wool on areas with hard to remove stains. Use fine steel wool. #000. The plastic is VERY tough.
Inside…the “secret sauce”…
Kids sandbox sand…”play sand”…very clean.
Put in about 2 or 3 cups. Add just enough water that it becomes a kind of slurry, like a milkshake. Put your hand over the opening and SHAKE! It takes a little time and effort, but it’s very effective at getting everything. It’s good because it gets all the brown residue off. There are no little orifices or hoses for the sand to get stuck in, so you can just rinse it thoroughly once finished. I was very pleased with the results.
I tried using a clothes drier lint filter brush at first, but that didn’t work very well.