Hi Jan,
I've not tested this exactly with the GC50 gelcoat but a couple of things jump out at me suggesting this would not work too well:
Firstly, the GC50 is designed for liquid epoxy to bond well to it, this is different from liquid polyester bonding well to cured epoxy (polyesters in general do not have good bonding strength themselves - GC50 works because it expolits the excellent bonding performance of epoxies).
Secondly, being a gelcoat, the GC50 is not designed to cure fully when in contact with air (i.e. the inside surface when it's been applied to a mould) and so you will have to contend with an unusual surface that would need to be rubbed off before you could start to flat it properly.
By all means give this a go but if it doesn't work (ideally on a test piece) then my suggested route would be to use our
Epoxy Coating Resin to build up a few layers on the surface of your tube, then flat and polish that surface (the coating resin is
very good used in this way and will stay completely clear even as it is layered over itself) and then finish the tube with a 2k lacquer or clearcoat.
I hope this helps and look forward to seeing the finished part.
Matt StathamEasy Composites / Carbon Mods - Technical Sales