You need to get the kits made specifically like that sold at paint stores and home depot. It's an epoxy paint system and prep is extremely important. Step by step instructions come in the kit. Just know all paint systems for tubs are temporary. Should last a few years to allow you to save up for remodel.
This bathtub finishing kit will work great: https://amzn.to/2Oxp9m6
Hope this Helps! Jeremy - https://do-daddy.com
Rust-Oleum does make a product that is approved for bathtubs.
Please consider looking over these links below and consider the Amazon website which has questions and answers along with reviews which may help you decide is the product is right for you to use. I have use the Rustoleum helpline they are very knowledgeable and educated people that might help answer your questions or concerns before you consider the project.