visionset wrote:
I'm sure I've seen mention of this technique in the archives, but can't find
it now.

I've posted that I've done it in the past.

Idea being with my very uneven gaps on softwood casement openers, to:

1/ Open window
2/ Cover opener contact points in suitable release agent (Cling film,
silcone or PTFE spray?)
3/ Seal on to frame in the proper position of a typical draught excluder
with low mod silicone.
4/ Close window to vent position
5/ Time passes...
6/ Open and trim with knife as necessary.
7/ Have lower bills and more comfort.

Anyone like to add anything to help task satisfaction, esp concerning
release agent.

I used low-tack packing tape.
As I could find no low-tack packing tape, I simply used ordinary packing
tape, with the teenyist bit of cornflour applied to it.
With 'Flora Buttery" over the top of it, as a release agent.

It's essentially impossible to get it very neat.
If you can get to outside the window, then it may be possible to remove
the excess in a neat manner.
(white spirit soaked rags help lots)

Of course, for really rotten windows, just omit the tape...