How to Use Tamiya X-21 Flat Base – The Complete Guide for Modelers

Turn Gloss Paints Matte • Reduce Shine • Customize Your Finish

Tamiya X-21 Flat Base is one of the most misunderstood products in the entire Tamiya paint lineup — and one of the most powerful when used correctly. Whether you build military armor, aircraft, trains, Gundam, dioramas, miniatures, cars, or RC bodies, X-21 lets you convert glossy acrylic paints into beautifully smooth matte or satin finishes.

This guide covers exactly how to use Tamiya X-21, mixing ratios, common mistakes, and the correct way to apply it for perfect flat finishes every time.

🔍 What Is Tamiya X-21 Flat Base?

Tamiya X-21 is not a clear coat and not a standalone paint.
It is a flattening agent — a matte additive designed to mix into glossy Tamiya acrylic bottle paints.

When mixed with a gloss (X-code) color, it reduces reflectiveness and converts the paint into:

Satin
Semi-flat
Full matte
Ultra-flat finishes

Important: Never apply X-21 by itself. If used straight, it dries as a frosty white powdery layer.

🎨 Why Use X-21?

Tamiya’s gloss paints are beautifully smooth, but not all subjects should be shiny. X-21 gives you total control over finish:

Flatten car interiors
Tone down shine on aircraft
Make armor more realistic
Create matte weathering layers
Flatten colors before decals
Create your own custom satin finish
Matte down figure clothing or skin
Remove sheen from wagons, boxcars, tanks, or railcars

It’s more flexible than buying a separate flat version of each color.

🧪 Mixing Ratios for Tamiya X-21 Flat Base

This is where most modelers get confused — and where mistakes happen.

Here are the surprisingly simple rules:

⭐ 1. For Full Matte Finish

Mix 1:1 (Equal parts)

1 part X paint (gloss color)
1 part X-21 Flat Base

This produces a beautiful, dead-flat finish similar to XF paints.

⭐ 2. For Semi-Flat / Satin Finish

Mix 3:1 or 4:1

3–4 parts gloss paint
1 part Flat Base

This gives you a soft sheen — great for car interiors, aircraft, figures, and rolling stock.

⭐ 3. For Subtle Sheen Reduction

Mix 10:1

10 parts gloss paint
1 part X-21

Ideal when you just want to reduce the factory gloss without going fully matte.

⚠️ X-21 Warning – Avoid Frosting!

X-21 will frost or turn powdery if:

Used straight without mixing into color
Mixed too heavily (e.g., 1:3 or 1:4 Paint to Flat Base)
Sprayed too thick
Applied over uncured paint
Not diluted properly before airbrushing

If frost appears, you can sometimes fix it by:

Applying a light gloss coat
Applying another coat of properly mixed paint

🖌️ How to Apply Tamiya X-21 (Brush Painting)

Mix your color + X-21 using the ratios above
Add a drop of Tamiya Paint Retarder 87114 for smooth brushing
Thin with X-20A or Lacquer Thinner (87077) for flow
Brush in multiple thin layers

You’ll get a streak-free matte finish that’s far more natural than a heavy clear coat.

🛠️ How to Airbrush Paint Mixed with X-21

Mix paint + Flat Base using ratios above
Thin 1:1 with X-20A or Tamiya Lacquer Thinner
Spray at 15–20 psi
Use light, even coats
Allow each coat to dry 5–10 minutes

Using lacquer thinner (87077) gives the smoothest flat finish.

🌟 Pro Modeling Tips for X-21

✔ Use it to flatten glossy metallics

Perfect for worn aluminum or oxidized steel effects.

✔ Matte down decals

Mixing X-21 into clear gloss creates your own satin clear coat.

✔ Use for weathering layers

Mix a small amount into dust, dirt, or rust tones to prevent unnatural shine.

✔ Perfect for model railroading

Flat Base is excellent for boxcars, locomotives, cabooses, and structures.