Simply use banana oil the same as cellulose dope, after the tissue has been shrunk.
Thin it 50/50 with cellulose thiners and apply it with a loaded brush.
When covering wings, do the underside first and then the top, give it a minute or so for the thinners to flash off a bit, then pin or weight down the edges exactly like you do when you water shrink.
Banana oil does not shrink the tissue, but stops any further shrinkage.
The tissue will go baggy in moist and humid conditions, but when things are back to normal it just goes back to where it was.
I once had a model land up in a tree for a week, where it was rained upon several times.
When I got it down with the aid of a friends roach pole, I simply put it on the shelf and it went back to the state it was before!
Supplied in a 125ml tin
SPECIAL DELIVERY NOTE
Banana oil is a flammable and therefore hazardous substance that can only be shipped within the UK using a specialist courier.
Any order from our website containing banana oil will be charged a shipping premium. See the delivery tab for details.