What is Reddit's opinion of Wet Belt Sander - Glass Craft Applications?
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I have a few thoughts:

Ideally look into something for sanding glass like:

https://www.amazon.com/dp/B00DP8QK4O/ref=cm_sw_r_apan_glt_fabc_4T2CYPVV9BPDB4H1MRZV

I really don't recommend using a hand sander how you described. but if you're going to do so anyways. Get something double insulated and don't let the water drip onto anywhere near the sander motor. (It might find a way anyways using the sander upsidedown)

If you're on a tight budget but have access to a drill. Maybe consider a headlight polishing kit like at an auto parts store. They're meant to sand plastic and the sanding pad can get wet. You will need to clamp down your work piece. And use the drill in hand.

3M 39195 Ultra Headlight Restoration Kit, Easy Heavy-Duty Restoration https://www.amazon.com/dp/B08745K56G/ref=cm_sw_r_apan_glt_fabc_SSFP988VXF5G6CENGCFX?_encoding=UTF8&psc=1