Hi. Could you seal and mdf pattern with sticky back plastic before laying up a mould? Seems a lot of work to pattern coat and flat sand a large flat area. I read somewhere that melamine covered mdf can be used but if I have already bought the wood could covering it in sticky back plastic work?

If it's a simple flat surface then vinyl wrap will do the job. It will struggle to adhere to any contour on mdf though.

Melamine faced timber is commonly used but it's best to sand the melamine to lose its slightly textured appearance. Most of our flanging work is done using these options