Hi,
i will expose my point of view on prepping a part to make a mold out of it. This is based on my experience, I'm not a master at all, so maybe other people will give other good advices.
Well, say you want to do a trunk lid for your car out of composites, say carbon fiber.
You have to take your trunk out of the car and prepare it. So you would need to roughly sand it but not so smooth so primer can adhere to it. , then apply a primer that will work with the different chemicals you use for making the mold. What I usualy do is to apply a good "thick" coat of primer (Given your part dont need to be precise at half a millimeter..). I always use polyester filler. I never had a "master part" sticking to its mold. But I do know from my experience than using primer that will not work with heat or chemicals you are going to use is gonna lead to a FAIL haha.
So my first advice: use a primer / filler that is compatible with your mold. Dont be tempted by rattle can primer ... I made that and once again it lead to a fail. I use polyester filler but people will tell you that polyester filler and epoxy mold dont mix good. I never had a problem tho. Easy Composites guys will let you know which kind of primer /filler you need regarding how you want to make your mold.
You will then need to sand the primer to final shape. Before proceeding, apply a very light coat of guide coat. i dont bother buying "genuine guide coat" rattle cans, I just use your regular rattle can matt black. You need to spray it more like a mist rather than a coat. Then you need to start making dust !! i usually start with 600 grit.
Second advice: always use a cross pattern to sand your part. Linear patterns will more than likely lead you to create "valleys" in your panel. Dont use your hand as a sanding block... you gonna end with valleys on your part. A simple rubber rectangle on which you wrap your sanding paper is a cheap way to sand more accurately. AND ALWAYS WEAR APPROPRIATE SAFETY GEAR. Dust in your body is a no-no.
Your goal is to make the guide coat dissappear. If some black areas are left when you're done making one sanding, that means you have some low areas on your part. You can then fill it as you want. I use bondo. Try applying bondo with some rubber spreader. That way you will have less sanding work. Once you're done with sanding your bondo, apply a new good coat of primer / filler. Then guide coat, then 600 grit paper. repeat process as many times as needed. Your hand is your friend. Feel your part with it, it will see more imperfecions than your eyes will...
Then I use 800, 1200, 1500 and 2000 grit paper to "polish" the part. I finally use polishing compound (without silicon please, silicone can mess your mold). After that, I clean part with acetone.
And there we go, our part is ready to make a mold. Expenses so far: like 60 euros, main part (with what I use as products at least) is the polyester filler. And, yeah, a good bunch of working hours too. But thats free.
And finally you can start making your mold. So, as opposed to what you were saying in your post, prepping a part to make a mold out of it is more " loosing time" if you cant get your part straight or whatever, rather than loosing big money. Building the mold is more a matter of being sure the part will demold (making barriers with plasticine so the part is not mechanically locked, using wax and other release agents etc). My experience would suggest that if you're sure about your part prep, you're more than likely being able to make a mold that will easily release. And if your mold came off your "master part", then your composite part will demold easily too, once again given that you use correct release agents.
You can start making small molds and parts with polyester and fiberglass, its far less expensive than epoxy and carbon. Then you can move to bigger parts or other mold types. I hope I helped you decide to have a try at it....POWER TO THE DIY'er !!!!!
Cheers from belgium.
A $1000 electronic device will always protect a 10 cents fuse