Both of these sprays, the Filler and the Styrofoam Primer, are, on the one hand, white paints and, on the other, primers that can be painted over. There are many reasons for applying a primer to a surface: 

• The substrate is sensitive to solvents (for example, Styropor rigid foam) - in this case the primer acts as a protective coat and barrier layer.
• The substrate is rather dark and you want to apply a light coloured paint like orange or yellow - in this case the primer acts to improve the efficacy of the paint. This includes the special effect of, for example, the Urban Fine-Art Phosphor(escent) spray paint.
• You need to optimize the adherence and durability of a paint on a substrate (like metal): The primer (in this case the "Filler") ensures excellent adherence to metals; the subsequent coat of paint makes an excellent bond with the primer.
• You want to improve the evenness and homogeneity of the intended surface because it is uneven or lightly scratched.

Styrofoam Primer: The Styropor Primer is a highly pigmented special quality paint for preparing sensitive surfaces for painting; these include  Styrofoam and polystyrene - but also wood, terracota, stone and other absorbent surfaces. It can also be employed as a surface sealant or insultion coating. The best results are achieved by applying a number of thin coats with five minute intervals.

Filler: The Urban Fine-Art Filler is a strongly adhesive primer for use on many different surfaces - for example, Styrofoam. Because of its somewhat high degree of viscosity it can be well used for filling scratches and cracks or pores. The primer can, when required, be sanded after five minutes drying time and, after being cleaned, can be painted over (use 600 grit wet sanding paper). As with the Styrofoam Primer, applying a number of thin coats with five minute intervals will achieve the best results.