It's finally time - the top coat! Open up the can of Interlux Gloss White Brightside Polyurethane Paint and follow the directions for thinning it with their specialized and expensive brushing thinner and either brush it onto the inside of the tub or spray it on with an HVLP gun and line dried compressed air.
Paint on two coats of the top coat letting the paint dry for the manufacturers specified time doing a very light sanding with 400 grit paper in between coats.
The finish that results from the Interlux Brightside paint is considerably better then what I would have expected from a standard enamel paint.
The paint was more expensive and certainly harder to work with since it had to be thinned before it could be brushed out well, but in the end it yielded a finish that you'd have to look really hard at in order to notice that it wasn't porcelain - just look at that shine!