All 18650 Li-Ion batteries are hard to solder. Well, not only 18650 but all nickel plated metal elements can be very hard to solder. But only if you do not have the proper tools. In this case, proper flux: soldering acid. Forget about sanding, scratching and other voodoo. Just watch the video, it will clearly show what I have in mind!
No more sanding or scratching of 18650 Li-Ion connectors. A drop of soldering acid sill all that you need!