I have some old door trims and I plan to use the backing cardboard to produce a mould - as a noob it's the first ever mold .  I'd appreciate advice for surface prep. The surface has a cross-hatch pattern like masonite. I plan to sand back and possible use a spray filler and sealer but would like advice. Tips and suggestions?

Is it actually masonite? I took it as just having a surface texture like masonite... If it's masonite then yeh, don't spray it with high build. Probably need to seal it first.