What do I do if I can't get the remaining wallpaper glue off my wall?

by Phil
(Sugar Hill, GA, USA)

I have a 10 year old house and I removed the wallpaper and chair rail from my breakfast room, including an 8 inch strip of heavy trim-type wallpaper below the chair rail. The upper section of wall came clean easily after removing the wallpaper, but the section under the chair rail didn't come so clean. Instead, I was left with a bit of glue that just won't come off.

I used solvents, scraped, and sanded with both a sponge and a flat hand-sander with 100 grit paper but it seams hard as a rock and won't budge. This has left me with a strip of light texture around the room and although I've applied primer (mostly to cover some darker paint used on the lower section of wall) and sanded some more, it persists. The texture isn't horribly deep or real defined, but my hand can feel it, and at certain angles the light brings it out. I'm worried that this strip of texture will be noticeable after painting as the rest of the wall surfaces are smooth with no real noticeable texture.

Is there some type of sanding technique or heavier grit paper or equipment I could use to smooth this strip of texture out, or would it be advisable to just apply one more coat of primer and then paint to see if in fact the strip of texture is really that noticeable?