Sticks that have not cured or seasoned are more likely to crack or split as they dry out. If the stick moister level is high the paint or any finish may have issues also. I am not sure what Willows you have in the UK. But you should be able to fine information on the willow in your area. You have a number of stickmakers clubs or guilds in the UK. I would encourage you to visit one of their meetings or shows.These groups are great are great places to learn.