...
Pink/XPS foam sand into needed shape. Foam is attached using hot glue. Coat in dry wall mud (even layer).
http://thecompositesblog.com/making-a-composite-part-from-a-foam-plug/
Sand to shape then apply filler. Patent coat primer(1 coats). Then sand again (120 grit) to get rid of high spots). Low sports with body filler. Patent coat primer(1 coat). Sand again. 240 grit on last coat.
Applies high gloss finish (for high gloss finish). After cure finish with a 400 grit area. Move on to wet sanding with an 800 girt paper. Change water and use 1200. Change water and use 1500.
Polishing compound.
Gel coat. And apply fiber glass.
https://www.epoxyworks.com/index.php/the-lost-foam-method-of-composite-fabrication/
...