Touching up Tough as Tile Paint
There's a link to the applcation instructions on their website.
https://www.homaxproducts.com/kitchen-bath/sink-tile-refinishing/tough-as-tile-one-part-finish-spray-on-kit-white (this page is not the instructions but contains a non-transferrable link )

It says not to allow it to cure for more than 6 hourse before recoating, so you'll have to re-do the surface preparation around your repairs or on the whole tub and then re-coat.  

I see no indications that it is not self-compatible - that is it can be applied over an existing tough-as-tiles finish if the preparation instructions are followed (again)