Hi I personally do the following.

1. Black undercoat. Always. 6mm -28mm  I always black undercoat. I've seen many who white undercoat but I prefer black. I'm an average painter and black, more so in 6mm, hides my sins. 

2. Washes  not me, I can't get a good result with washes in any scale  so I don't bother. 

3. Varnish. Always. Matt usually but i see the appeal in gloss. 

4. No. the appeal of 6mm is that I don't have to worry about buttons. My hussars get a few white/yellow/black lines to look like lace but that's it. 

If you're worried try painting a few spare strips in different styles and see which is easier for you  

Hi Oppi

I find you can get great results using a few simple techniques, and that it's a matter of what you want to paint. My figures always end up being good enough for me to love them, and other people like them. I use a variety of washes, stark highlights and I'm pretty happy with the results.

I did a blog post on how I paint my Napoleonic French - here:

http://aleadodyssey.blogspot.co.uk/2016/10/painting-napoleonic-french-in-6mm.html

Average painter here.

I use a grey primer and then undercoat in black.  I block paint in bright colors; dark blue I use light blue etc.  I don't use washes on 6mm. 

One thing I've learned, at least for my liking, is to not paint too much detail.  I do test units to see how they look from a distance, and sometimes painting every belt, scabbard, and ribbon can actually detract from the look of the unit.  1812 Russians, for instance, I skipped the white crossbelts in front because I lost to much of the green coat at eye distanc from the table.  My basic figure is hat, coat, pack, roll, gun, cuffs/collar, maybe scabbard, and plume/poms/cords because they are on top.  Oh and skin.

These units paint up fast for me and look cool in mass formation on the table. 

1. I use both depending on the figures. White is good for bright colours but black creates a nice contrast. For white undercoats I give them a black wash so I can see the details better

2. No, not enough paint layers in this scale.

3. yes 2-3 coats

4. yes I try to paint details like cheeks and noses on the faces, and also distinctive thinkgs like cuffs which help identify the figures.

1. Black primer every time.  I like to block paint, and you can't do that without black primer.  Leaving a tiny bit of black between different colours (e.g. the pants and the shirt) really makes the minis pop.

2. No, washes muddy the figures and require rework to bring it back up.  If you block paint as mentioned above, you don't need them.  Primer, basecoat, one highlight, done!

3. Always, matt varnish for me.  I paint on Daler Rowney soluble matt varnish and it works like a charm.

4. As much as possible yes.  Definitely not buttons, but facings, straps, etc. yes if I can.