Black Hexflex paint.
Hexflex is a water-based, fully flexible, durable brush on paint devised for foam surfaces. It is Latex free, won't crack & cures with no tack.
Also available in white & clear (see below).
Hexflex is simple to use. You can apply straight to foam surfaces (you may want to heat seal first), but you can also apply to almost any surface including polystyrene, wood & fabrics. .
Hexflex is supplied thick, but can be thinned to the desired viscosity with water.
It can be thinned enough be used with an airgun.
*Please note: drying times can be considerably improved (20-30mins) with a gentle hot air source (heat gun or hair drier etc). Drying times are estimates and may be quicker or slower depending on local conditions.