This is a way to get no "ring around the tub" when you're pregnant and the belly bump gets in the way of your bending over to clean the bathtub. Simply buy a cheap broom from the Dollar Store and use it specifically to "scrub/sweep" that tub sparkling clean. A little shampoo squirted on that broom will get rid of that oily, dirty ring around the tub. Then, turn on the shower to rinse. By the way, this also works for me now that I'm much older and have difficulty getting on my knees!
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A stiff broom with plastic bristles makes a nice tool for cleaning the bathtub without having to bend down. Scouring powder works well with the broom. Of course, if there is an unusual amount of hard water deposit, it may still be necessary to scrape it off with a knife.
Now that I'm an Oldie but Goodie I have found an easier way to clean the bathtub. After my shower, I lightly sprinkle cleanser in tub. I got a broom from Dollar Tree, wet it and swish it all over, tub and walls.