I got one like that once, and I used an old facial scrub-pad to lightly rub the pesky areas after I had damp cloth-ed the hell out of it. The facial pad was rinsed to make sure no scrub-stuff was still in it, and I just used it damp. I figured if you can put it to your face, it would probably not hurt the guitar any, but I did not go at it like I was killin' rats, either, ya' know? Nice and light, and though it took a minute, when I was done there were no unwanted scuffy marks left from the pad. I have taken sandpaper to necks in the past, sure, and I most likely will again, but NEVER as a means of cleaning debris from one. As far as scrub-pads go, if it leaves no marks on a pane of glass that's great: it may work for you, BUT if it leaves no marks on a plexi-glass/acrylic panel, it should definitely do the job without harming your finish. Just don't go at it like you're killin' rats, and closely monitor your progress, and that nasty old neck should be sparkly, squeaky clean in no time. Worked for me, but, as always, YRMV.