An alternative is to use paint-on grippy materials. You can purchase these materials in cans from most home improvement stores (usually in the paint section). If you want to go with this approach, be patient and paint in layers. Trying to paint one single thick layer usually ends up with uneven coating and affects the roundness of the finished product.
Silicone and latex sealants (also called caulks) also have lots of traction. Unfortunately, these materials tend to be very thick. It is difficult to apply these materials onto the wheels evenly. Again, one trick is to use an attached power drill as a lathe to help evenly apply sealant. Unless you need sealant for other purposes, you can purchase the more expensive pressurized cans for easier application. You can also purchase the toothpaste-like tubes so you don't need use a caulk gun. Try to look for sealants that ``remains permanently flexible'' so you got both traction and suspension.