Hello,
In the past I have resurfaced a cast iron bath and some hand basins. I have done this using  2 or 3 coats of gelcoat..
It has been fairly successful apart from a few chips and yellowing over the years.
I am about to do this again on another cast iron bath ( which will be lightly sand blasted first ) and thought that this time I would first give it a coat or two of fiberglass resin then a coat or two of gelcoat, hoping that this would be an improvement.
Could you offer any guidance on this ? Thank you.

keep in mind that a gelcoat wont fully cure on the "airside" gelcoat is made to be used when making mould (on the part side), then covered with fiberglass for example to reinforce it.