Soap Scum build-up....Hmmmm?
You don't have to re-invent the wheel...
I will say it again, and again..MAGIC ERASERS by MR. CLEAN combined with a degreaser (simple green) or oven cleaner from Dollar Tree. You will need to pre-spray the ring around the tub, let it sit for a few minutes, then start at the top of the shower/tub.
Spray your scrubbing bubbles, go back down to the ring around the tub, elbow grease with a magic eraser... the ring should
wipe right off...you may feel a pull from the scum, no biggie, just keep going around til it's gone.
Go back to the top, and use the old karate kid...wax on left hand wax off right hand, have a damp warm microfiber
rag handy and wipe off excess in a figure 8 motion...all the way down to the drain. You will be shocked!!! This has worked
for me for years, a little bit of trial and error, but finally I have mastered it.