Alternative to Paper Mache

Inflatables are going to change in size so I don't think it's possible to have a clear hard shell that remains on the item & lets the design show through. Are you planning to use an inflatable as a (one time use, disposable) mold for a weather-resistant piece? If so, you could use quick setting cement and patch over the areas that inevitably crack while the inflatable is slightly moving and changing in size. If it becomes too cracked, you could spray over that with spray plastic/acrylic and see if it works. However, I wouldn't invest much money into this because there's a good chance that nothing will properly stick to an inflatable in one piece without using layers (like paper mache.)