How do you clean a bath tub that hasn’t been cleaned for a year.

Hah! I've got you beat. Mine hadn't been cleaned for three years. No different from any other cleaning. The stuff just does not get any uglier or more tenacious over time. It would be helpful to know if it is an acrylic tub since you would need to worry about scratching if you used an aggressive cleanser. I put a coat of car wax on my porcelain steel tub when it was finished and it did not make the surface slippery. Just wanted to get that last little bit of anything on the surface removed to like-new.

Easy Off No Fume Oven Cleaner in the BLUE can. Spray it on and let it sit till it liquefies. Rinse. Works on enamel, porcelain, and fiberglass.

Hire a maid.

I would fill it with hot water and Oxyclean. Let the water cool and rinse well. What doesn't come off with a Mr. Clean sponge should be removable by Bartender's Friend and the sponge.

Oh dear, not a pleasant chore. I would spray heavily with vinegar to start, then a baking soda/vinegar paste or barkeepers friend.

My best cleaning friends are Krud Kutter and Magic Eraser.!!

I've used Toilet Bowel cleaner on really bad tubs & shower walls. Need to use gloves & mask for safety and work quickly. It will take it off easy. Rinse well.

start with white vinegar and water,spray and leave on over night,Use Mr.clean Magic Eraser,after that is done see how much more residue is left,make a paste with baking soda and water next,leave on again until it dissolves the grime,at that point you should be able to use a regular Lysol bathroom cleaner with peroxide

William's advice sounds as though it would work on a really dirty tub that hasn't been cleaned recently.