Primer is useful to allow the paint to better stick to the miniature. I would either spray prime or brush prime, depending on circumstances. I live in a flat, so spraying stuff is always difficult. Thus I brush prime.
A protective coating after the paintjob is done is very important. I would use matte varnish. There is no realistic alternative to the spray can here, IMHO.
I use water based varnish, which I paint on. I'm not an expert painter (and not that artistically talented but I try...) but I find that the little effort of varnish helps hold the work I do.
Spray on work BUT is prone to clouding issues depending on weather and humidity. Living in Canada these days means I either kill myself with the fumes inside or spray varnish for a couple of week in the year - as it's either too hot, cold or humid to always come out right... So paint on varnish is safer.
For general gaming a Windsor Newton or similar matte varnish will work fine. Use the stuff you don't have to dilute. Just brush it on lightly and make sure it does not pool. Thick coats don't help and may in the end spoil your paint result.
No problem. Everything on my blog that I paint is varnished - either with the 2 coat technique or just WN matte. As I mention a light coat of olive oil get rid of cloudiness if you ever get it. But avoid thick coats of paint on - you don't need it.
I do need to get back to Ancients/DBA painting...been spending a lot of time on WW2 this year though and board game figures.
Depends on the amount of bashing you figures will take. If lots of bashing then spray gloss varnish first - I just use Humbrol for that...but beware if using enamel metalics then they can run. If using acrylics you'll be OK. Then (or if you will look after figures well) then spray matt varnish. Testors Dullcote was the "industry standard" if you can find it - but this was reformulated 5+ years ago to remove all the CFCs and atmosphere rotting chemicals and IMHO it don't work so well any more. I now use "The Army Painter - Anti-shine Matt Varnish which is good stuff.
Now if you are really lazy and get the technique perfected you can combine Warpainter Matt Varnish, with "The Army Painter - Quickshade" which is a oil based tinted gloss varnish. This seeps into the folds and creases of your figures to add shade, depth and texture to the figure....then Matt Varnish over the top and jobs done. This works really well on 28mm figures and I have seen it used to good effect on 15mm but haven't tried it myself yet (I'm old school and have only recently migrated to acrylics. The basics are - basecoat, block paint colours, quick shade and the Matt Varnish. Google "The Army Painter" to see the technique in action.
The important stuff with spray varnish is to shake the cans very well and use sparingly in a well aired location, outside, garage etc. Don't spray when very hot or very cold conditions and spray from about 9-10 inches.
I used to use a lot of enamel paints so used an enamel matt varnish with a tip of a brush if matt enamel black in it to slightly dull the colours.I've never liked the "parade ground" finnish as wanted my armies to look like they have been "in field" and weathered.I even used to choose campaign dress figures for my Napoleonic armies where choice was available. If using gloss or metalic colours I would varnish them with a gloss varnish first and then use the dulled matt.I chose this method of finnish as I am not a great painter and the highlighting combined with this dulling effect looked ok for me.
Now with a lot of Hobby/model shops vanishing I use GW citadel paints more and more,so settled on Lamian medium with a slight mix of Nuln oil gives me a simular effect to what I used to do which is handy when re-touching old paint jobs or matching new figures into old armies.