TL;DR:

• when assembling models, you should fill any gaps before priming
• small <1 mm gaps = Vallejo Plastic Putty
• large >1 mm gaps = Milliput Standard

Vallejo Plastic Putty is excellent for smaller <1 mm gaps. Apply the Plastic Putty along the gap right straight from the tube. Immediately after, lightly wipe away excess with a dry finger, a paper towel, a cloth or a toothpick. After that use a damp cotton swab to clean excess further away from the gap. You may sand it further after it has dried and this stuff dries fast. Repeat until you can drag a toothpick across the previous cap without feeling a bump. Water-diluted Milliput Superfine is also an option but more messy to use and requires you to get the consistency just right. Citadel Liquid Green Stuff is too chalky and stiff, but could be used with dilution.

Sprue goo (leftover sprue mixed with plastic cement / acetone) can also be used to fill small gaps but it can be a bit messy. Works nicely to add character on organic shapes though.

Milliput Standard is perfect for larger >1 mm gaps where runny putties would be hard to use. Green Stuff and Tamiya Putty are also ok but require stronger liquids for dilution like lacquer or alcohol. You can dilute Milliput with just water.

Some people like to use 50/50 mix of Milliput + Green Stuff, but I personally don't think it's worth the hassle in gap filling. Maybe for sculpting.

Milliput comes in multiple variants, but I usually just use the Standard for large gaps and sculpting; and Superfine for small gap filling if I'm out of Vallejo Plastic Putty tubes.

I find Green Stuff is too stiff to be optimal for gap filling, but it's a great material for sculpting and conversions though. But it can be used to fill gaps.

Make sure you seal your Milliput sticks airtight, or it could harden while in storage.

The process of filling gaps with a putty:

• Prepare the putty according to its instructions.
• Shape the putty into a roll akin to the shape of the gap you are filling.
• Push the putty into the gap with a sculpting tool i.e. a toothpick.
• Optionally use the sculpting tool to texture the surface as you like.
• Let the putty fully cure.
• Optionally carve, file and sand the area.

In general, you can use anything that air-dries to a solid to fill gaps. The rest is largely just convenience and personal preference. Don't use 1) things that don't actually cure like Play-Doh or 2) things that are oven-baked like polymer clay.

You can use any matte varnish or Mod Podge to fill gaps at a pinch.

Sources

• How to Paint Citadel Miniatures, David Cross and Rick Priestley