I tried using Krylon Flatt Finish, but this has or appears to have a darken effect although not always.
I can say this is because my tank which I painted dark yellow with green and brown camo turn 30% darker after being sprayed by this product.
Until today I'm not sure why this happen, either it is a reaction between my arcrylic paint to the krylon or with my oil wash. However, this spray can does produce a dead flat finish.
Other that I tried is Testor Lacquer Flat. Mixed with 3:1 Flat and thinner and sprayed with my airbrush.
The finish is not as flatt as the above, but does not produce any gotcha.
It does however, fade the color slightly, so control is very important.
I have not tried mixing the Tamiya XF-21 base flatt with anything, but since it carries very thin white-ish residu, the final result maybe like the Testor Lacquer Flat.
Until today I have not find any flatt finish that I really like.
Mr. Hobby has a flat spray can product, but I have not tried it yet.
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Ben