A still raw piece can deteriorate. A fired piece will not dissolve or alter unless the piece is submerged for long periods of time in high-temperature water. In any case, you can moisten a surface to apply transfers since it is a momentary exposure that you will dry in a prudent time. Transfers are made with special transfer papers.
If what you need is to soften a dry and hard clay, you must use specific solvents for polymer clay, in drops that you will add according to the amount of clay to soften, and interspersed with the use of the rollers so that these solvents integrate well into the clay. mass. This process will recondition the material.
On the other hand, if what you want is to preserve your piece from water or friction because you have painted it or applied volatile powders, you can use self-leveling varnishes that seal your material, or epoxy or UV resins, the latter being the most easily manipulated. as components or cured.