Also, how to soap scum-proof a bathtub transfer bench.

might try 'Rain-ex, but it could make the bench very slippery --- just use a good tub/tile cleaner when ever you clean the shower/tub itself

I use white vinegar, and wipe, the smell will go away when dry, it also stops mold

can't think of anything that won't make it slippery.

There is not magic wand that will make it "soap scum proof". Only regular maintenance- cleaning, and scrubbing with a brush, will prevent it from accumulating. It is usally easiest to do this immediately after bathing, when any soap film is still wet and hasn't hardened yet.

What I have found to work best is to spray the chair along with the shower walls with something like "Shower to Shower" or "Daily Shower" after the shower is complete. Then they are easier to clean every few days and the scum doesn't accumulate.

You need to wipe it down after each shower to get soap residue from drying out. Vinegar will help cut the soap scum, and the antibacterial and antifungal properties will keep it sanitary. Let vinegar final rinse stay on the seat and dry. There is no way to keep soap scum off the shower seat, it needs to be rinsed and wiped down well each use to prevent bad buildup.