Good morning all,
As part of a student project I have to make an infusion flax fiber boat hull.
Having a fairly small budget, I have to make my mold myself (4 large molds 2500mm * 1000mm * 600mm).

I chose to make a rigid polyurethane foam mold that I would come to cut in order to have my shape.
However, I don't know what product to use next to waterproof my mold and obtain a smooth surface effect.
I saw that the S120 (https://www.easycomposites.co.uk/s120-advanced-board-sealer) might be suitable what do you think? what about duratec 707-002 (https://www.castrocompositesshop.com/fr/appr%C3%AAts/1557-1323-duratec-707-002-appr%C3%AAt-polyester-gris-fonc%C3%A9 -compatible-% C3% A9poxy.html # / 403-weight-495_kg_100_g_de_pmec)?
What products do you use to make your molds? Have you ever made polyurethane molds?

I also thought of putting film of stabilizer for model making oracover type but I do not know if it adheres well to the surface.

In any case, thank you for your advice and I hope to have feedback from you

**Sorry for the traduction I am french and english is not my best asset.

Unless its a high density model board as opposed to a foam, chances are the S120 will not have the filling abilities to fill the surface pores on a typical foam.  Ideally a thin scrim of woven glass and a good quality coating epoxy,  such as our XCR Coating resin, should give you a usable surface to work from.  It should be slightly more durable than Pattern Coat Primer and other pattern coating materials as they are generally designed for a single use for mould making. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant