The roof that i'm thinking of using GRP roofing on is overlooked and i'm wanting the best finish possible, how can I get the smoothest possible finish?

The best way to get a super smooth finish to your GRP roof is to use something called surface tissue, a very thin fibreglass reinforcement that is applied with the base resin.

The product is really difficult to apply, especially on a windy day and a lot of patience is required, but once it's down and dry, sand off any high spots or snags and apply the finishing topcoat as normal.