Hi Andy. I found that the sails I had in my Kyosho Farwind kit had been folded. I hung them out but the folds persisted. The material is a man made fibre so should respond to limited heat application.
I turned on the iron to a low setting and then laid out the sails on an ironing board. I covered the sails (in part) with a wet handkerchief and ironed over the handkerchief until it dried out.
The wet handerchief is only there as a temperature indicator.
If the folds are still there, wet the hanky again and repeat. Keep on doing this until all folds are gone. Mine came out after several goes.
I was worried that if I used the iron too long and the sails got hot that they might shrink.
There were no problems using the above method, just remember that as soon as the hanky dries out the temperature can rise above boiling point very quickly.
Hope this helps,
regards Roy