This has been discussed before here in the general consensus is that it's not worth the effort, and possible danger of a simple mistake. A single milligram or two of missed fentanyl could result in death. It's not worth it.
I'm sure people here will tell you to throw the drugs away, and they're not wrong. If you don't want to do that, volumetric liquid dosing, or mixing everything together in a soluble liquid REALLY WELL, and then drying it out would remove the hotspots for the most part. Start with a REALLY small dose and go from there.