Guitar and bass strings can exert hundreds of pounds of tension on a guitar neck. So, it makes sense to slacken off the strings when you're not using the instrument, right?
It's certainly been mentioned a few times around here that the strings on your guitar exert quite a bit of tension on the neck. Enough, usually, to pull the neck into a 'bow' shape (with the middle being farthest from the strings). Our trusty truss-rod can generally be called on to counteract that tension and to control the bow or even straighten the neck completely against the string pull. Sometimes, however, we get what's referred to as 'back-bow'. With a back-bowed neck, that bow shape is effectively reversed and the middle has ended up closest to the strings. This means that a note fretted in the lower end of the neck can't sound clearly as it hits off the 'uphill' frets all the way to the middle of the bow.
There are dual-action truss-rods available and these are able to correct for back-bow as well as, the more normal, forward bow. They're becoming more common but lots of guitars still have the usual single-action rods that can only correct forward bow to counteract string tension. As this is what it needs to do in 99 out of 100 cases, that's generally fine.
But sometimes it's not.
Setting up this guitar, I found it was back-bowed beyond the point where string-tension would have corrected it. Open notes and those as far as the eight fret or so all choked or buzzed. Even with the truss-rod slackened off completely, the neck wouldn't pull straight or into relief (a very slight bow).
There are a number of things that could be done to try to address this, some of them relatively involved jobs. Before getting into those discussions regarding what isn't really a super-expensive guitar, I had a punt at a quick-fix. This doesn't always work and isn't always suitable but it's certainly worth a go in this case.
In the first photo, I've got my heating blanket sitting on the neck, weighted down by a heavy fret-leveller. I don't want to go nuts with the heat here. Too much will just cause hassle in this case but I'm applying sufficient heat to slowly heat the glue holding the fingerboard to the neck. I don't want it to completely let go; just to soften slightly.
When I think it's where I need it to be, I clamp the neck into a bit of a forward bow and leave it to cool. If things go to plan, the glue will re-harden and help the neck to hold some of the shape I've forced it into.
And, luckily, things went to plan. Unstrung, the neck now has just the slightest of back-bows and, under string-tension, that pulls into a little relief—enough to get it playing nicely again.
Incidentally, a workplace safety tip: Even if you are fully aware that this procedure will leave your steel fret-leveller in a very hot condition and, even if you wear heavy gloves to move it, you shouldn't put it over to the side of your workbench, right where your elbow will touch it when you're checking clamping pressure. That would be stupid.