I observed that drying before grinding was not a good option for making a mangosteen powder. There was no problem during the drying process but the problem rose when grinding commenced. The soft wet mangosteen rind turned to hard wood structure after few days of sun drying. Manual grinding was near to impossible. Motor operated grinding is feasible but it needs heavy duty metal burs, components and a high horsepower motor. Definitely not suitable for home-based processing.
The easier method in my mind was the grinding before drying. However, grinding might pressed out considerable amount of juice together with precious xanthones. Thanks to stainless steel shredder! I managed shredding several fresh mangosteen rind with ease (without any expelled juice), just like shredding cassava for cassava cake making.
update as of March 2018:
I recommend chopping or slicing to manageable sizes and sun drying. Nearly powdering it is an option but filtering them out after boiling and before serving will be hassle.
Rind should be dried before storage. Keeping it wet inside refrigerator won’t prevent mold growth.