Pirate sword painting
Do you mean paint won’t stick to aluminum foil? Because any two-component epoxy based paint will stick to aluminum and aluminum foil. But I think your approach is flawed from the get go. You should look into the way cosplayers build swords. They have a skeleton-like frame (sometimes a solid wood dowel) underneath the sheathing which is very often a type of foam matt like you would find at a camping store for underneath a sleeping bag or a type like that which is used for yoga. They cut shapes and glue them together, perhaps even screwing them to the dowel, paint them, apply decals from model kits etc. The videos can be quite entertaining.