When soldering platinum to gold always use cadmium free gold solder. For health reasons, this is the only solder you should be using anyway. However, for platinum it is especially important. Cadmium can migrate into the grain structure and cause brittleness.
Before joining platinum and gold together ALWAYS finish and polish the platinum piece. Platinum requires a longer polishing time than gold. If they are polished after joining the gold would wear away before a final polish could be applied to the platinum. As platinum does not oxidize when heated, the polished finish will remain after soldering.
Platinum and gold contract while cooling at different rates. Because of this, if you have a long solder seam between gold and platinum the seam may crack as it cools. To minimize this try to design your jewelry creations with a short seam where the gold and platinum touch. If a long seam cannot be avoided, after soldering cover the piece to allow it to cool slower. A used tuna fish can make an ideal cover. After soldering place the can over the item you soldered and warm with your torch, then allow to air cool slowly.