Originally Posted by: Guitar Tech

Originally Posted by: hsnoeckx

looks to me like it is just the oxidation layer which is rubbed off, not really cavities under the strings, probably your guitar has been hanging in the store a long time without cleaning, something you should do to after every time you played; just wipe the strings with a dry soft cloth.

hsnoeckx is exactly right - you're looking at corroded fret-wire. Removing the strings, followed by a thorough polishing with 2500-grit sandpaper on a 'short-block' (this is a hard rubber block, that's made specifically to hold a strip of sandpaper) will make short work of this problem. However, Do Not use any sandpaper coarser than 2000 grit, for this job - sanding frets with coarse sandpaper will result in randomly lowering the frets, which would be a disaster!

Stephen White

Thanks!

I'm glad that's all that it is.