Debugging why the vacuum bagging hadn't worked, I finally concluded that my mold itself was not air tight and no amount of tape at the edges was going to seal it. A bad vacuum bagging results in pools of hard plastic at the low points of your part as it sits in the mold, and lots of flashing. Cosmetically, this is devastating (if you care, I don't really in this case). Functionally, it's weight that doesn't make the part stronger and can mess up fitment. So for next time, I decided to change technique to enveloping the entire part, mold and all, in bagging film like a big empanada. The first opportunity to apply this was to top skin positive attempt 1.
Carefully weigh the cloth.
Some maths to figure out the right amount of epoxy ingredients.
Success! Vacuum bagging achieved!
Except this time, unrelated things went wrong. That carefully measured amount of epoxy? Apply a fudge factor next time. So I had some dry spots instead of pools this time. I had run out of peel ply and switched brands to the place I was buying the tooling gelcoat, and the pore size on these was much bigger so more epoxy immediately ran into the batting instead of spreading out. Maybe they expected the user to be much more careful about spreading the epoxy super evenly first, but on a mold with high and low points this seems impossible. The resin is going to want to run.
Attempting to wet out the dry spots is probably not what an F1 team would do, but no way I was going to throw away something this close to right. Second trip through the vacuum bag and I couldn't get it to seal. Seriously? I must have torn it somewhere when I opened it the first time. I wonder if I can get more durable bagging film designed to be reusable or if this is just a fact of life.
Anyways, the top skin came out frumpy but close enough for the 50-50 rule: 50 yards away at 50 mph, it'll look right.