Silvering

The Tamiya Flat Base is an additive to make a gloss paint dull. You NEVER ever directly apply it to a model.  

A sequence that I (and many modelers) use is:

1. Gloss coat (e.g., X-22 or Future)

2. Decals

3. Gloss coat to seal them

4. Washes

5. Flat coat.

I apply Future, place a decal on **still wet** Future and seal it with Future. With this method, it's impossible to get silvering.

You can mix X22 and X21 (3:1 or 2:1 ratio) to make your own flat varnish, but it's just easier to buy an acrylic flat varnish made by W&N or Vallejo.

X-21 Flat Base is an additive which is mixed with gloss colours to make them flat (or anywhere between). It is not a clear flat coat and should not be used as such.

XF-86 is a ready to use flat coat. I haven't used it, so I don't know how well it works. 

Any gloss clear coat will work to prevent silvering. It does not need to be a specific brand or product.  Then, if you want a flat final finish, there are many flat clear overcoats available.

Thank you for your confirmation points.

I have just messed up with a direct application of  X-22 Clear and a subsequent coat of XF-86.

The streaks (pencil applied) of the X-22 are visible through the XF-86.

Time to use tigermans dot filter :)

Happy Modelling!

Zack