On to the first attempt so far at making a positive, using the ribbing.
Vacuum bagging... more on this.
Ribbing positive attempt 1 came out wrong in several ways.
- Not enough carbon layers, didn't really know how much I should have used.
- Again with the improperly mixed epoxy. This stuff is not forgiving. Use a scale, mix two minutes like your life depends on it.
- Vacuum bagging failed in multiple ways.
The vacuum bagging process could take a thread all its own. You put down the carbon cloth, pour epoxy over it and try to spread it around with a roller or brushes as evenly as possible, put peel ply release film over that, put batting over that to help with air flow, put this bagging film over all of that, use gummy tape to stick it down to the flanges of your mold in an airtight seal, attach a vacuum pump or venturi like mine to a port, and the 20-25 in-Hg of vacuum makes the air pressure evenly squeeze your part. The epoxy should spread out in an even layer and the excess comes through micropores in the peel ply and gets absorbed by the batting.
What actually happens is that you chase phantom air leaks infinitely until you're hallucinating with rage and frustration. I tested my equipment on some little swatches that turned out right.
Attempt 2 of the ribbing positive also failed to draw enough vacuum, but mercifully came out good otherwise. Properly mixed, enough layers to be rigid, etc. Super light. Hilariously light.
There was unfortunately minor release damage to the mold. Localized. Probably fixable.
Not perfect, but this is a success!