It's really hard to say much at a distance, but I'll add this. You don't say just how old this mixer is. If it's very old, like several decades old, wire insulation can break down and flake off, especially if you disturb it. Then, of course, it may leave bare wire that can short against the case or some internal part. It is of course shorting, which is the source of the spark and the thrown breaker.
I'm guessing you took the covers off to paint them. Take them off again, and look at the insulation of any exposed wiring. Look as well for any wire that you may have pinch while putting it back together. Old wires supplying power to motors are often brittle due to the heat of the running motor. I see it a great deal with fine old electric fans, the ones made back when they had real motors. If you find that the insulation has broken down, I would suggest taking it to someone comfortable working with electrical devices, so they can replace the wire or repair the insulation. Repairing motor wiring can be tricky, because the wire may well run all the way down into the motor windings, and it may be difficult to replace the bad portion.
Such a person can also check by meter for any other short to the case, which is important, even though you are using a ground fault interrupter that we now know is working.
If you find that kind of problem don't fret about it being your doing. If it is so old that the insulation is breaking down, it simply its age, and you may have discovered a problem that would have surfaced eventually, perhaps not while plugged into an interrupter.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen