I've seen a ton of different "recipes" for cleaning products and I finally got around to trying one for a tub scrub and was blown away by how well it worked!
I couldn't remember the measurements exactly but I just got a bowl and squirted in some Dr. Bronner's Lavender Castile liquid soap, then 2/3 cup of baking soda, then a glug of white vinegar. Mix it up, scrub it around, rinse, and be impressed!
I actually hate recipes like the one I just gave... a squirt of this, a glug of that... I like precise recipes, the more details the better, but this one really is okay if you just eyeball it. However, I did read a tip about how you should never mix the soap and the vinegar together first because they'll neutralize each other (which is why you can use a vinegar rinse to get rid of soap scum).
So I put the soap in the bowl first and made sure it was covered by the baking soda then glugged the vinegar on top. It fizzes up but settles down quickly and then I just stirred it up and poured it into the tub. No more elbow grease needed than usual and my tub has seriously never sparkled like this before. And I had no need to worry about breathing in toxic fumes or absorbing them into my skin as I scrubbed without rubber gloves. And no gritty feeling the next time I got in.
I'm almost excited for the next time I get to scrub the bathtub and I never ever thought that would happen :D And in case you're interested, here are 51 more uses for baking soda!