This metal may be used for ballasting of plastic models and balsa flying models. You simply melt the solder in hot water and pour it into a convenient void - ideal for adding weight to locomotives, wagons and carriages. It should not melt or deform the plastic as long as you heat it in this way. The metal expands slightly on cooling which makes it very suitable for holding punches for multiple holes, and for holding awkward shapes while they are being worked on. Even plastic items may be embedded in it. It can also be used for making simple one-off castings. Use Plasticine as a mould.
What we call whitemetal is usually referred to as Low Temperature Alloys for Centrifugal Casting. The melting rage varies from 125 - 285 C, so you really should be able to undo a whitemetal kit by immersing it into a bath of very hot water.