if you first paint a layer of resin on the inside of the mould you get no bubbles. i am a sculptor and use fibre glass you could use a gel coat and you can also tint that and paint it up in layers, i have made transparent objects 1/2’ thick. i use regular oil paint and mix with uncatalysed resin and then add that to the resin to pour up, this way the colour is mixed even and to the strength you want. you inspired me to make up some big fins for summer. thanks