when you say "plane down" the wood, do you remove wood from the thick side or the narrow top?
From the wide side. So it will always be 20mm wide but taper from 3 to 2mm for example. Thats essential, if you do it the other way, the batten will tend to break.
When you apply epoxy before glassing. Do you let the epoxy dry, clean and sand, or do you put on the glass before the first epoxy layer have hardened?
I let "dry" (wait a couple of hours), but do not sand. The idea is that the wood soaks the epoxy. If you do not do this first, you may have some dry glass, because the wood is like a sponge and takes the epoxy from the glass fibre. Also the wood is protected from moisture.
Have you noticed any lasting twist/corkscrewing of the battens if you leave them in a rolled sail?
No. If yes, you wood takes moisture, hence it wasn't fully encapsulated in resin.
How lang have you sailed with wood/glass battens (durability)?
Three years on my Dart 18 (then I sold the boat) and now two years on the T. I sailed the Dart every week-end in the spring and summer for a couple of hours. The T is sailed less.
We sail in strong winds as well (25kts).
One small thing. If you tie your battens in the batten pocket it might be good to drill the hole oversize, fill with thickened epoxy and re-drill once hardened. Otherwise you might get cracking and water/moisture seeping in there.
I used a small brush to get epoxy in the hole. Less professional, but did the job. Or a notch on the end instead of a whole.
Cheers,
Klaus