Robert if you use the plastic spacer between any joint the filling is going to be minimal. Good thing about this approach is that you only have a superficial surface fill as the joint has been filled. You do not need a filler to fill a structural joint which can rupture.
For these joints I use Mr Surfacer 500. I brush this along the joint in a very thin worm.
http://www.scalemodelshop.co.uk/manufacturers/albion-alloys.html?p=9 This pages in the shop has a great range of mini brushes which are perfect for this work. Clean up brushes etc with IPA.
Depending on the depth of the joint to be filled a number of applications may be necessary to build up level. Between each application you can wet and dry, scrape gently with a Morton knife to level or use IPA with a brush to flatten the filler or just fill on fill. Each coat takes about 30 mins to dry. Nice thing about this filler is that it is about the same softness/hardness as the plastic. Wet and dry the joint and it will flatten the filler with out the edges falling away. It feathers absolutely flat to the plastic edge. Provided it is carried out well after priming you will not see the joint.
Warning after it is primed, using acrylics, the dryers in Mr Surfacer will act as a paint remover if applied over the paint film
For small imperfections and making good after priming I use Vallejo putty filler. Not putty in the sense of putty for glaziers. It is a thin smooth paste. The container comes with a fine long cone producing 1mm diam worm to place the putty exactly in the place you want it. I use a wet finger to smooth the putty into place. After it has hardened you can use IPA to smooth it bearing in mind that IPA is also a good acrylic paint remover in quantity. Provided you are careful you can get away with it. Or use Vallejo thinners which is not so destructive.. While wet or soft water works well.
For very fine blemishes when in the bare plastic stage (before primer) I sometimes use a coat of Mr Surfacer 1000 especially good over Vallejo Putty to give a very smooth finish. It is so thin you do not have a perimeter edge to worry about.
Best to try these out to see if you like this way of filling.
Laurie