On most recently built sails the sticky back numbers come off fairly readily. However if the sail had been exposed to summer heat (we keep our's in the cool basement during the summer), the sticky back may have become more gooey.
Here's the trick:
Remove battens.
take the sail out-of-doors where there is plenty of fresh air.
Spread out area with numbers on plywood sheet ( put on high horses, it'll be easier on your back.)
Soak affected area of the sail with methyl ethyl ketone or lacquer thinner (this is why we're working out-of-doors)
Peal off numbers and goo with plastic putty knife.
Wipe away the residue with paper towels (lots of towels required)
Soak and wipe again as required.
Use only as much foul language as is required.
Burn the soaked paper towels out-of-doors (do not discard in a trash can, fire hazard)
#2 was not possible in the cold weather
#3 ping Pong table worked for that
#4 used Goo Gone,very little help but non toxic.
# 5 found only my thumbs rubbing it worked.
#8 too late ,I actually invented some new foul language (i think it might be french)