I did this with one of those citrus-oil based crayon removers, Goof Off is one brand, I think they're all the same.
Works great, wipes off clean and non-oily so the next one will adhere well.
Use Goo Gone....Goof Off will actually damage the helmet as it contains acetone. Made that mistake with a flat screen television I hung in a corner. Thank goodness it was an old one.
1. Peel off old decals with fingernails as best you can.
2. Wash helmet with wet rag to get dirt/mud/grim off
3. Squirt of WD-40 on a rag. Wipe the rag on the old sticker spot. Use liberally as needed, just be careful not to spray on padding as it will cause the helmet to smell like a oil change place.
4. Use a clean rag to remove the WD-40 when done otherwise new decal will not stick.
Thanks for the suggestions. Anyone have anymore suggestions I'd appreciate it.
I told one of the coaches in our org that has replaced old decals that if he demonstrated how he did I'd buy the pop. I told him I usually need to watch something a dozen times before I get it down though......
I actually googled it. It seems that people do it from football helmets and bicycles. My son is a graphic artist and I know that he would heat it with a hair drier first and then, pick it off. On YouTube I saw a guy with a pencil eraser on a drill and he erased the remaining residue.
I would try Krud Kutter first. If that does not work, I would try rubbing alcohol. Goof Off/Mineral Spirits would be my last resort as it may dull the finish on the helmets by removing the polyurethane.