For this build I decided to jump in with both feet and learn how to build fiberglass. I found out early on that I would need to build up a custom air dam so I decided to build out a door as a trial to learn the process. I have done plenty of small FG projects but never something large that involves building a mold tool.
You can see the process in the pictures. First sanding and waxing the door (plug) and applying mold release. Then a layer of gelcoat and then laying down multiple layers of heavy matt until the mold has enough strength and rigidity not to break when pulling the final part.
The only mistake I made was not smoothing out the original door. I thought it looked fairly straight but it turned out to have some waves in it. It also had some small flaws I thought would be easy to correct in the mold. I was dead wrong. It would have been much easier to correct the flaws on the original door than to work on the mold. You can see the final surface on the mold.
Then I prepped the mold, laid in some layers and a new door popped out. It's hard to tell in the pictures but the surface finish turned out great. I added some cardboard rolls in the back to make it stiff. It is attached to the car with push button locks and pins on the bottom. For the final touch I 3d printed a door handle.