The only really good way to re-enamel a bath is to do just that - shot-blast all the old stuff off, then fire on a new vitreous layer. It's hellish expensive, and only worth doing if you've got a pretty fabulous old cast-iron bath to start with.
I've tried most the proprietory kits available in the UK, and without exception, they're crap - and I don't imagine the bath renovation stuff in the USA will be much different. You may get a reasonable match in terms of translucence and colour, but it doesn't wear well, and it's virtually impossible to avoid getting texture problems.
Mind you, the nicest people have baths with bare patches!