Are you trying to simply make a paintable dash cover using fiberglass products? Is it a one-off piece, or do you plan on making more than a couple of these?
On that salvage dash you can bondo it smooth, then primer it with urethane 2k primer or polyester primer like FeatherFill primer, then sand it smooth to a glass like gloss finish. How? Simple. Wet sand it with 400, then 600, 1000, 1500 paper. Then take a buffer ( not a car polisher as that is a different animal) with buffing compound to it until it shines. Careful not to burn through the primer with the buffer.
Now use some pure carnuba or mold release wax ( 12 coats, waiting 30 minutes in between each coat) and lightly buff off each coat using a microfiber cloth. You know, those super soft car rags sold at WalMart and the Auto parts store. Take your time and do this waxing correctly and you will actually be able to seperate the new part from the waxed dashboard. Also the wax won't substitute for poor sanding and polishing so make sure the primer is glass smooth and has a nice shine prior. You need all three here: good sanding, good buff job, 10 to 12 coats of wax applied correctly. You'll be so happy when you can release your new part and see your handy work accomplished.
I would also recomend using vinyl ester resin, not polyester. polyester is not as durable to the suns effects and seeing as this is a dash going behind a glass windshield the sun will be beating on that dash. Vinyl ester resin ( VER ) also shrinks less than polyester resin. Home stores, hardware stores, and automotive suppliers sell polyester. Composite supply stores sell VER. It is used the same way and mixing method as polyester. VER is also good up to 235F degrees! polyester is typically good up to 150F or so. Most new boats now are made with VER, sometimes refered to as "marine resin". All these resins you catylize with 2% mekp: 12 drops per 1oz of resin.