How to repair a chipped bathroom cast iron tub?

They sell "paint on" products at home improvement and hardware stores. I just did a couple knicks in my kitchen sink.

Home improvement stores carry porcelain repair. The one I used comes in a bottle kind of like a nail polish and has a little brush. Just brush it on. My repair lasted at least 10 years.!

Does it really match the color of the tub?

I had a black, elongated chip/shard on the top edge of my white bathroom sink. It stood out every time I glanced in the bathroom. After cleaning the sink one day I grabbed a bottle of plain old pure white acrylic craft paint from my stash and gently brushed some over the damaged area. It looked okay so after letting it dry, I applied a few more thin coats to cover; being careful to keep the paint smooth and only within the chip. (If left to cure, acrylic paint becomes permanent). I let it dry but the matte finish looked dull. I grabbed my bottle of clear nail polish and gently brushed a few layers on, extending just past the edges of the paint and allowing it to dry between coats. Knowing me, I used a blow dryer to hasten dry times. Be sure to allow surface to cool completely between coats. You know, that repair was done out of irritation and without much thought but it actually looked pretty good. The color and sheen matched and on close inspection was barely visible; so much less visible than that big black spot (that always looked like a spider to me). End of story: I knew this was a temporary fix but not to be the case. We sold that house several years later with that same repair as it looked the day I did it.

I wouldnt use that tub paint over stuff on any tub u intend to use. I've done it on 2 different tubs. It flakes off and u bathe with porcelin chips in your water.....now have to find a product to remove that epoxy stuff!