The Panda (like most cars these days) is fully galvanised at the factory, so should need no further rust treatment of any sort. The 'paint treatment' is often a 'posh wax' that will wear away... Personally I wouldn't bother.
Best way to prevent anything under the car rusting (suspension parts etc) is to hose accumulated mud etc away from under the wheelarches etc, especially in winter when it contains salt, and to be sure the drain slots under the springs and in the subframe are clear. That will cost you nothing but prolong the car much better than wax (which tends to block up those crucial drain holes)