You're right, there's really no such thing as Cold Cast on Sintra because CC is when aluminum powder is mixed into resin and cast in a mold. The advantage of sintra is that its easy to work with, light weight, and is great for homemade armor. Fiberglass is sturdier and is meant for casting complex shapes or reinforcement. It holds its shape very well. On the other hand, if you leave your sintra armor in a hot car, it may deform.
Done right, Rub n Buff can look very close to Cold Cast. I have CC FP Gauntlets and my RnB'd armor matches the CC Gauntlets very well. RnB and CC don't "go odd" in color in time, but they may lose a little bit of their shine. The CC can be polished with aluminum polish. The RnB can be buffed again and may eventually need to be re-applied.
I don't think either CC or RnB have a problem with rain, although I suppose some of the RnB could wear off if consistently subjected to rain or if the RnB is wiped hard when wet.
