Ray, Michael,
Thank you for your suggestions. Ray’s use of the term ‘release agent’ sent me down a Google hole which gave me all sorts of interesting results. Here’s a clear Swiss explanation of what epoxy won’t stick to
https://acrylgiessen.com/en/what-does-epoxy-not-stick-to/. This led me to sheathing tape which in turn sent me to eBay. And it turns out that I already have some in my shed. Described as sheathing tape, an obviously very specialist item, it costs £12.99 for a roll. The exact same item described as Gorilla clear repair tape (which I already have) is £8.50. The power of marketing!
I’ll use Gorilla tape to make a mold in the gap between the outboard well perimeter and the ply infill. I’ll then fill the mold with epoxy, which will only adhere to the infill. Wait for the epoxy to cure, peel away the tape, lift out the infill and the gap should be thoroughly closed, with a bit of luck. I’ll post before and after photos here in due course, whether a success or failure.
Michael’s cyanoacrylate activator looks to be interesting stuff and is now very widely available. You can get a tube of superglue with a can of activator for £4.98 from Toolstation.
