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.

Yellowing over many years is usually down to slow UV damage. White shows this up the worst.  Most Polyesters are good in terms of UV protection, especially compared to epoxy resin systems. You may find as you are going for the most sensitive colour - white - that you need to look for a gelcoat that is specifically formulated with high UV resistance.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

To further what Warren has said, the type of gelcoat with the best UV resistance and the best long term resistance to water ingress (osmosis) would be an isophthalic gelcoat. This type of gelcoat is strongly recommended in boat hull construction above and below the waterline where UV resistance and resistance to osmosis are a primary concern and when used in this application (often white in colour) isophthalic gelcoats offer very good stability over many years. I would say that a bathtub faces far less demanding conditions in terms of UV and water exposure than a boat hull which would lead me to expect very good resistance to yellowing or deterioration when used in this application.

Our Superior Quality Polyester Gelcoat is an isophthalic gelcoat, as described above, and so would be very good for this application. This gelcoat can be pigmented using our range of Polyester Colour Pigments which includes a White Polyester Pigment.

Matthieu describes the effect where polyester and vinylester gelcoats do not cure properly when they cure in contact with air (air contact cure inhibition) which is in fact designed-in to polyester gelcoats so that subsequent layers of resin or gelcoat can crosslink properly with the first layer, forming strong interlaminar bonds. In your case, on the last layer of gelcoat you apply, the addition of a small percentage of Solution MW Wax Additive will prevent this cure inhibition from happening (by creating an invisible barrier between the gelcoat and the air) which will allow the gelcoat to cure fully, making it much easier to sand and polish to a good finish.

I hope this helps and look forward to supplying you.

Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales