Is there an epoxy resin suitable as an external waterproof and hard surface coat that will provide a decent finish on EPS?

Hi flyerdave. For coating over EPS we would recommend our XCR Epoxy Coating Resin which is a great resin for over coating many materials including EPS.  It is highly UV stable and slightly thicker than a typical laminating resin thus making it easier to apply a thicker and even coat.  It is also easily sanded and can be polished back to a high gloss should you wish to further finish the part.

I would also recommend considering adding a layer of woven fibreglass to the EPS as well as the XCR resin to add further durability. EPS as you know, is relatively weak and although a layer of XCR will add durability to the part, it is still not massively strong.  An additional layer of woven glass can improve the durability of the part to impact.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant