What is the bathtub made out of? What type of finish would you be applying to the bathtub?
Wish I could be of more help but I never wrote down the mix. I think it was 50 % white I, 40% slag, 10 % vcas and then some pva fiber and 70 mesh quartz sand in moderation. I think the mix was 1/3 sand and 2/3 binder. It was used to parge a window well foundation bellow grade on the old school house about five years ago. Mix water was about 10% polymer. Can't remember which one.
I used a Duraamen product called "scraffino" on shower walls and it still looks good two yrs later. It comes in 3 grades. Regular, fine, and super fine. the super fine has no sand at all. The idea is to use each one, starting with the regular, to keep filling in until smooth. It's usually used on floors as a microtopping. Maybe it'll work.