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 StathamEasy Composites / Carbon Mods - Technical Sales