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?

Don't take this as a recommendation as I am a beginner to but this is a mould I made recently.
The sides of the mould were too low (long story) so in desperation I shoved the parts back into it and used car body filler to build the sides up. I then sprayed it with lots of rattle can lacquer. I'm pretty sure I broke at least half a dozen rules in the making of this mould but after spraying it with PVA I was able to pull a part from it without damaging the mould at all. In fact I will be using it again over the weekend and hopefully several more times.
Like I said don't do like I did but it seems you can get away with quite a lot if you use plenty of PVA,

.

Spray with high build, then flat it down with 400. Then spray with PVA, nice and thick so your part goes blue. 

In my experience of working with the brown Masonite I found that body filler and glues etc do not stick to it. I don't know what is in it but beware and maybe test it a bit before throwing too much time and money at it?

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.