For this step, just get some isopropyl alcohol and clean the exterior really, really well.

Clean the tumbler's exterior with isopropyl alcohol, and lay out enough aluminum foil to cover the tumbler like a burrito. Lay a shop towel or paper towel on the foil, and saturate the shop towel in acetone; it needs to be pretty wet. Tightly roll the towel around the tumbler and then roll that tightly in the aluminum foil, and set a 20-minute timer.

In about 20 minutes, pull the tumbler out of the burrito, and retain the acetone-soaked towels (we can reuse them for paint prep). Use a plastic scraper to remove the bulk of the paint (I used one that came pre-loaded in my 3D printer), and an awl or razor blade to remove the small bits in cracks/joints or small parts.

When you've got all of the finish off, scuff the surface with a Scotchbrite pad, and make sure you get any surface etching out. Clean the exterior of the tumbler again with the acetone towels to make sure there's no residue.

Wearing gloves, ask off the areas you don't want to paint, and be sure to mask off the opening so you don't get paint inside of the tumbler. If you get fingerprints on the tumbler, make sure you clean them off with acetone before you paint.