I have a pretty large area to cover and I think we are only able to get the GRP roofing resin and glass down on the first day. I want to know if I can apply the topcoat the next day or possibly the day after that?

Yes, it's possible to leave the roof up to 24 hours before applying the topcoat, just make sure that you cover the roof overnight and to make sure it's completely dry before applying.

Sand the roof first either with normal 60 grit sandpaper or to make the job quicker use an orbital sander. Make sure you have a face mask and safety glasses on while sanding and always work against the direction of the wind.