How do I clean years of built on soap scum and grime on the bathe tub?

Make a mixture of baking soda and white vinegar into a paste,apply to the soap scum and leave on overnight

Some depends on the tub. Acrylic requires care that porcelain or cast iron do not. Standard answer I see is to use one of those Magic Eraser products. Won't hurt the acrylic. For grimier porcelain or iron, there are products similar to Soft Scrub that will really get in there.

A painter I used recommended Krud Kutter to clean years of grease build up in kitchen. The stuff is amazing and I've used it all over the house. I find it in Paint Department of Home Depot. It's safe and can be used without gloves. Comes in spray bottle and gallon containers. I would use it with a Mr. clean magic scrubber. Hope it helps.

I used 5 tablets of polident tables (cleaner of false teeth) soaked in the tub drain follow up with the magic sponge and then vinegar..

try steam.

Buy a wet/dry screen , it's near the sheet rock section at Home Depot. Keep it wet and no need to use any cleaners... Works without much effort, just keep it wet and scrub away that scum. It won't scratch your bath tub either.

Saw Nicole Curtis of DIY network- say she used vinegar and baking soda on antique tiles she wanted to keep in one of her Detroit make-overs...

Easy Off Fume Free Max (don't substitute another brand or formula) oven cleaner, let sit for a little bit, scrub with brush, rinse very well. If its a fiberglass tub, seal and shine it with https://www.walmart.com/ip/Gel-Gloss-GG166-Fiberglass-Polish-Protector-Bottle-16-oz/100775275

i had this same issue with fiberglass tub in the home we moved into. I tried everything. I saw a post where someone said use oven cleaner. i bought the no fiMessage easy off and sprayed the shower walls and tub. Let it sit to do its magic All I had to do was wash it down. No scrubbing and it removed all scum build up. Looks like a sparkling new tub and shower

use the limeaway toilet bowl cleaner

Use vinegar - spray it and scrub with an old toothbrush.

Heat white vinegar then add blue dawn 50/50. Saturate and scrub. If you have the patience, you can use Wet & Forget Shower and it will get it clean but it will take a bit of time. I used it twice a week instead of once (as directed) and it took about 3 weeks to get my no slip tub clean in all the crevices. You could probably use it daily. Just make sure to apply after showering. I had a friend try to use it otherwise and for some reason it doesn't work nearly as well. Perhaps it's the heat?