The main function of a varnish is to protect the paint job. If you plan on playing with the miniatures you are painting, you should apply varnish on your finished miniature. If miniatures are only for display, varnish isn't usually required. All uses:

1. Protective coating; to protect the paint job.
2. Improve decals; hides the decal seams.
3. Change the perceived reflectivity; matte, satin or glossy.
4. Make the next paint layer flow more; gloss helps washes flow to recesses.
5. Make the next paint layer flow less; matte helps paints to stay on surfaces.
6. Making "magic washes"; mix the varnish with some paints or inks.

Varnish doesn't have to be the last layer. For example, if you are using metallic paints, applying a strong matte varnish on top of that kills half of the idea of metallic paints (the shine). Consider adding varnish before applying last metallic paint highlights or alternatively varnish your metallic sections with a more glossy varnish.

Non-metallic metal (NMM) can be matte though.

The whole model doesn't have to use the same varnish. You might want to varnish some parts matte, some parts glossy.

Make sure your model is fully dry before applying a varnish. At least one hour as the paint must be dry on the surface and inside. Note that washes, texture paints and oil paints dry more slowly.

You can apply varnishes in various ways: brush, airbrush or spray. You frequently have to thin the varnish down with water on an airbrush. Otherwise, it can settle into the recesses and cause undesirable frosty look

You can use any varnish as a gap filler. Not as efficient as putty but does the job.

Varnishes have different levels of reflectiveness.

• Gloss: Finish with shine, darkens colors, the most durable.
• Matte: Finish without shine, dulls colors, fairly durable.
• Satin/Semi-gloss/Semi-matte: between gloss and matte.

Most professional artists prefer matte because then the environmental lightning doesn't affect less how the model looks.

The reflectivity matters:

• Matte allows you to paint like it will appear on pictures, but then you have to paint even the minute lightning details yourself. A blessing for display models, a curse for speed painting.
• Satin helps you to cut corners because the minor reflection from the environment add natural highlights.
• Glossy is useful for special effects e.g. to make surfaces look wet, organic or otherwise shiny.

Varnishes also have warmth. Matte is cool. Glossy is warm.

Varnish Reviews (from the most matte to the most glossy):

• AK Interactive Ultra Matte Varnish
  • Reflective like an eraser.
  • They are not kidding about the "Ultra Matte" part, and pleasant to touch.
• Army Painter Anti-shine Matt Varnish
  • Reflective like a crayon.
  • Result is more or less what most miniature paints look after drying.
• Vallejo Matte Varnish
  • Reflective like human skin.
  • Even though it says matte, it is slightly more glossy than without varnish.
• AK Interactive Matte Varnish
  • Reflective like human nail.
• Vallejo Satin Varnish
  • Reflective like metal.
• Pledge Floor Care Finish
  • Reflective like glass.
  • Runny, thin and milky-white before drying glossy, transparent and a bit sticky.
• Citadel 'Ardcoat
  • Reflective like glass.
  • The most similar to Pledge, but a tiny bit more glossy and thick.
• AK Interactive Glossy Varnish
  • Reflective like a damp surface.
  • Exceptionally runny, wouldn't use this with a brush.
• AK Interactive Satin Varnish
  • Reflective like a moist surface.
  • Easier to get a glossy effect with this than AKI Glossy as that one is too runny.
• Vallejo Gloss Varnish
  • Reflective like a plastic wrapper.
  • Pools quite a bit without thinning.
• Army Painter Gloss Varnish
  • Reflective like a hard candy.
  • Slightly yellowish before drying, dries mostly transparent and doesn't pool.

You can also use epoxy and UV resin as a varnish, but I haven't tried those.

Pledge Floor Care Finish

Pledge is a water-soluble, acrylic, clear, thin and sturdy floor coating. Also referred as "floor wax" but is not actually wax. But you can also use it as a cheap and super durable gloss varnish for miniatures!

My Pledge bottle was on the corner of the table for a week and didn't notice I had spilled some stuff under the bottle. It took serious scraping to get it all off.

Pledge has many variants; clear, slightly yellowish and milky. All dry transparent. All smell quite nice, but they are chemicals, so you shouldn't sniff them. May irritate skin.

It is formulated to avoid pooling, as it is used to wax floors. It has very low surface tension, virtually never forms droplets or pools up as it dries. It dries to satin glossiness and slightly sticky. It dries fast, 15 minutes or so.

You don't need to thin it down, but you can. It is seriously good as it is. You can even mix Pledge with tiny amounts of paint to create satin washes (usually black).

Pledge is especially excellent in varnishing glass-like surfaces. Dip clear part, wick off the excess and let it dry. Makes it look less like plastic and more like glass.