Hey Corvus,
this is how I would go about it, I\'m assuming the vehicle is ready for mounting
1. decide on location of vehicle, placing pins as required
2.build & decorate base as you would normally. Leave to dry completely as the next few stages will be making a bit of dust which you will need to be able to either brush or blow from the scenic part
3.starting with 180 grit, rub in the SAME direction as the grain to remove any overpainting glue etc
4.with a damp NOT dripping sponge or cloth wipe off the area you just sanded, leave to dry (about 10-20 mins should be ok) once dry it should look & feel furry
dont worry this is normal, what you are doing is \"raising the grain\" if you didn\'t do this with the water, the varnish would
5.repeat with 220
6.repeat with 320 \'ish
7.repeat with 400 (if you feel it needs it)
8.apply varnish, lightly sanding with 600 between coats, it should come up like glass

Alternatively you coul do the sanding up to the 220 then give it a light varnish (diluted 50/50) leave to dry 24-48 hrs, a light rub down with 220 again. Do your basing, carry on with sanding/varnishing
hope this helps, it is easier than it sounds