ウォッシング。黒茶エナメルを約10倍に薄めてジャヤブジャブ洗う。

WASHING. I mixed enamel flat black, flat brown and thinner 1: 1: 10 for washing.
エナメルシンナーを含ませた綿棒で擦り取る。少し残すと汚れた感じになる。

After hand-paint the wash with a wide brush, leave some time for it to dry, then rub away excess enamel using a cotton swab. Be gentle, otherwise the base paint will be rubbed away too. Leave some of the enamel paint deposits to suggest marks left by rainfall in the dust and dirt.
雨跡、こんな感じ。

Like this...
上から下へふき取る。雨が垂れる方向に行う

Move the cotton swab in one direction, from top to bottom, following the direction in which the raindrops would fall.
BEFORE

AFTER