John,
Glosscote and Dullcote are thinned with white spirit (a refined form of turpentine) and should probably go over the Future ok, but the best thing to do would be EXPERIMENT FIRST. Take an aluminum can or an old model and paint it with a color coat, then the Future (maybe apply a few decals before the Future just to see if your model will be safe), then apple the Dullcote mixed with Glosscote. Try to replicate everything you did when you painted your model, and check the results.
Sometimes I paint a can or some card stock when I'm painting my model, and from there do everything to the card stock or the can before I do it to the model (if I'm unsure how a particular paint will react to another) - better safe than sorry.
I've used dullcote on numerous occasions and am happy with it, but I've never really been satistied with Glosscote because it's always seemed to stay a little tacky - it never seems to dry completely - when you run a finger over the Glosscote covered surface, there's always a slight "drag" - meaning the stuff isn't completely dry.
Future will always dry to a very hard surface and it lays down very well. I prefer Future to any other gloss.
Cheers
LeeTree
Cheers
LeeTree