Cracking job Sir,
As for under coating,a light coat wouldn't do any harm.What you'll find is a normal solid coat you'd apply to a metal or plastic figure.can lift the upper fibres and fur the surface in odd places,and unsealed edges.Its unpredictable its seems to be down to the age of the card,and how densely pressed the fibres.also how damp the card gets with spray paint.
An even coat requires several light coats, applied over a couple of days.Even then you'll find 9 out 10 will work,and then you'll come back to a fluffy model when it dries.
I normally paint straight onto the card and matt varnish if I'm out to make a model for someone else,If its for me I don't bother.
The only draw back with painting directly onto card is it takes longer to dry than you think,The surface will dry and the sub layer will be damp.You'll find over handling will cause little 'grey' mottled patches of ware,if you handle it to much before its totally dry.( Its only an issue if you rush your painting of base colours)These patches will take paint over them again without ware.A layer of varnish does pritty much what it does to a metal figure.
Its easy to keep on top of,By touching the 'dry' paint.If it feels slightly colder than the room temp but dry.Then it's not.Simple rule looks dry+feels cold=still wet.
It doesn't mean you have to take days to paint your model.Its simple wet paint wipes off

.Its just sneaky paint when its applied to card .
I'll stop the frightening you off now and point out I painted and based mine in under two hour's.
On a lighter note now, you've made a traction engine.You'll look at those mdf dumpster's and barrier's and think I could have made them out of card
