Hi rob,
Loving the work so far. If I may give a little advice though?
First up, regards fixing the powders, some hair spray is a good option to try. It seems to effect colour a little less than varnish.
Another point that comes to mind, why not try mixing and matching between powders and paints? Both Humberol and Phoenix do fantastic ranges to play with, in a range of finishes. The 56XX in my avatar photo is one example of a mixed media. Another is using a small dab of Humberol Metcote Gunmetal stippled onto a buffer head to replicate grease marks. It has just enough shine to look the part, a similar method can be used on the edges of gangway connectors.
I would also suggest you try and take your pics in more natural light if at all possible! It shows the subtleties of your work infinatly better than artificial light.
Keep up the good work my friend, please keep the photos coming and I hope I dont come across as being negative or picky!
Sean