There are two degreasers that I use on old scales. The most common one is starter fluid. It is quite noxious and should be used outside, preferably with a very light breeze. It works very well. The other one I use is oven cleaner. Once again very noxious. with this stuff I often put it in a bag and let the scales soak for the day or overnight. When you first take them out and rinse them you will think there is a crazy cool color effect on the scales. Unfortunately it goes away fairly quickly. It will leave them quite clean and ready for filling and gluing. Nasty stuff both of them, oh and I forgot some gloves is a good plan too.