That’s an unglassed EPS blank in the picture. The paint is a waterbased acrylic. I use Nova Color but any water based acrylic will do. Mask off your panel. Pour the paint on. Squeegee the paint off with single uniform lengthwise (nose to tail or tail to nose) passes working from the center to the rails. Wipe your squeegee clean after each pass. If you squeegee over the same area it will muddy the effect.
As I stated in the original post, I don’t think you’ll get the same results on the sanded hotcoat because it is not pourous enough to absorb the paint.