was wondering if anyone could give me some advice my fibreglass bodywork comes up great after a polish but a few days later it becomes misty in patches all over its quite old ,
any suggestions welcome

Epoxy does absorb moisture so it sounds like this is what is happening. Is it kept under a close fitting cover?

I doubt very much that it is Epoxy. Mouldings are never left finished in Epoxy because it is not UV stable and will degrade rapidly. It is always finished with a coat of paint or varnish.

I would think it is almost certainly Polyester. What is probably happening is the gelcoat has become porous and when you polish it the voids become filled with wax and look great, for a while.

The only real solution is to cut it back with 600 up to 1200 wet and dry, and then compound progressively back to a shine. Assuming or course the gelcoat is thick enough to take it without the laminate starting to show.

I have had similar problems. I am hearing then that the long term solution to this is to use proper primer, many coats of paint, then clearcoat with automotive clear. My white body parts turned gray in no time, and I ended up going flat black and vinyl (that's not a great solution either if you don't have the proper stuff).