Varnishing is generally a matter of taste. You basically have two choices of type - paint on, or spray, and 3 choices of finish - gloss, satin or matt. I'll briefly go through each, and give some pointers doing it carefully:
Paint on - Messy, time consuming and generally more gloopy on the model if you aren't careful with your brushwork. Brushing on can sometimes dissolve the paint if you aren't careful and so 'smears' the model. Sounds crap huh? Why mention it then? Well mainly 'cuz u can use it to pick out details - for instance if you only wanted certain parts of the models varnished - gloss on gems etc.
Spray - Quick, easy, even finish if done carefully, but generally more expensive than buying a huge tub of liquid varnish. I personally use either
GW satin spray or Army Painter Matt spray and have found both of these to be excellent, if a little pricey. Maybe others out there can recommend alternatives?
Satin - Generally a slight sheen to the model, but not unpleasently shiny compared to gloss. I do 90% of my models with this.
Matt - Great for mutes schemes, a must if you have painted by using the dipping method. Leaves models with a matt finish.
Gloss - Leaves models shiny with a glass like surface finish. Generally hideous in terms of whole model varnishing, IMHO unless that is the finish U are looking for. It is great for finishes on shiny armour, gems, glass etc.
Care -
Ive already warned above of the dangers of over zealous brushing - just a couple of words about sprays. You need to spray in an even temperature. If it is too hot, or too cold the varnish may go 'white' and spoil the finish of your model. This happens most with 'matt' vanish. The same can be said if the environment is too damp. Also - if you can't spray outdoors, and need to do it in a shed or garage (i don't recommend the bedroom, kitchen etc unless you want a fine coating of varnish dust over everything) then keep the room as well ventilated as possible and maybe wear a mask.
Finally don't hold the spray too close to the model. It will get a dusty, whitish finish if you do. Best thing to do is put the models into a large box lid, lie them on one side, then use several short bursts from different angles, moving the can quickly from side to side so as not to spend too much time coating too much of one part of the model. When it's dry, flip over n spray from the other side. Once dry again, pick up the model and check it from all angles to see if you've got even coverage. Generally spray from about 12" away. I often finish by putting a plastic bag over my hand, grasping the base of the model and giving short quick bursts into all the awkward angles, and hard to reach spots.
Hope the advice was useful.
