What is some advice for painting a bath tub?

Rustoleum makes a tub and tile refinishing kit. you might want to give it a try!

There are products out there, but nothing really lasts, you could have some one come in and spray it, or we had the Bathfitter come in we had a new overlay installed over the tub and a surround over the old. It was pricey but so worth it. https://www.bathfitter.com/locationfinder

There is a glaze you can put on the tub to resurface it. Check a local paint store.

https://dengarden.com/remodeling/A-Homeowner-Guide-on-How-to-Paint-a-Bathtub

There are kits at your local home improvement store. But if you want the finish to last a while (or if it's a high traffic tub), consider hiring a company to do it. The stuff pros use is more toxic when applying it but lasts a lot longer.

Rustoleum make a product in spray and roll on formula called Rustoleum Tub and Tile Refinish. Limited colors though-about 20 a quart available at most home centers.

We watch a lot of Home and Garden TV. There is a show on there called Rehab Addict. They send bath tubs out to be glazed. Those tubs look brand new after being glazed. That is the advice I would like to give to you: send your tub out to be re-glazed. You won't be disappointed. Best of luck to you.

I've done this and honestly wouldn't do it again because the products are very toxic and it took days for the smell to leave. However I did like the results. I used a white spray by Rustoleum, purchased at Lowes. This was recommended by the sales person. If you use a spray over a brush application make sure you cover the entire room with plastic because a residue travels. Paint with the door shut and windows open, wear a mask and work fast. Once finished closed the door for I think it was three days. Spend time researching product before you dive in and watch for dripping because that's where I ran into issues. Once it drips from the spray can onto the surface it's hard to remove it.