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Very specific question:

I have one fret that's high, I want to heat it and give it a few taps so the space underneath the fret edge is largely gone, then do a fret level.

If I heat the fret, the glue will weaken. Will the glue continue to stick after the fret cools down again?

I guess it depends on the type of glue and the temperature...?

I’ve never heated a high one. I just tap it down and if it doesn’t sit down, I remove it, clean the slot and install a new one.

For high frets on guitars in for repair the process is to tap the fret back down or clamp it back down and see if it stays there. If it does I clamp it down again and run a small bead of thin CA on one side of the fret and if I'm lucky I will see it wick up on the other side of the fret. Wipe off excess and it's also a good idea to wax the board on both sides of the fret with a thin coat of Howard Feed-n-wax being careful not to get it into the fret slot where we want glue to set. Once the fret is down, the board waxed in the positions on either side of the fret and the bead of glue deployed and I've watched it wick away into the fret slot I hit it with accelerator.

Now if the high fret or an end that is not down is because the fret was improperly installed either by you or someone else and it's "sprung" meaning that it's been bashed too hard with the hammer and/or it's radius is now less than the board's radius replace the fret. When I fret or replace a fret I radius the fret wire a tad tighter than the board radius which encourages the fret ends to lay all the way down once the middle of the fret is down.

We press our frets with the SM Jaws II tool and our plethora of radius cauls made by Andy Birko and then glue them in place with thin CA on a waxed board to make clean-up easier.

If you are replacing a fret a fret dress should follow but before that every single fret should be checked to be sure that they are all down and glued in place. If one was bad unless there is a reason specific to this one fret others could be bad too.

As for glue being reactivated I would not bother with that the goal is to seat the fret (or replace) properly and have what results be all the way down.

Also of interest and related I can't take a pic of it right now but my business partner Master Luthier David Collins built a tool for seating frets that is based on a spring loaded punch that you press and then the spring releases and the business end is driven forward. He created a mini-fret caul on the business end. We can press this against a fret and when we press hard enough it releases with a loud bang... and drives the fret home. If the fret will stay down or not is always a different matter.

Hope this helps you out.

The fret is high in the middle, the fret ends are touching the fretboard just fine.

I figured I'd have to heat it, so the ends can move a bit sideways as the middle is being pushed down.

Makes sense, no?

No heating is not part of how we seat a seat a fret at all. For the record we operate a very busy commercial shop in Ann Arbor and service around 1,100 guitars annually not counting many mandos, basses and banjos. But don't tell anyone..... We are Martin certified and were Taylor certified until we dropped Taylor's warranty work.

So I am providing you with professional advice from a couple of guys who have done this for decades now and have repaired successfully tens of thousands of guitars now.

We don't ever heat a fret to seat it. If the middle is not seated tap it down with a fret hammer and see is it holds. I'm not concerned that the ends will have sub surface level barbs moving sideways, that will help keep them down, just what we want. If the middle does not stay down once tapped down clamp with a caul and run CA as suggested on a waxed board to aid clean-up.

If none of this works it's an indication that someone else tried to seat the fret before and there is nothing left for it to bite on below where we can see. In that case I remove the fret (I heat to remove) clean out the slot and shape and install a new fret, glue in place, check all the others, glue maybe if there are other loose ones an do a fret dress.

When removing a fret let the beveled jaws on the fret nippers gently "lift" the fret, never, never pull up on a fret or you will be chipping wood out of the fret slot and fret board. Fret nippers are specifically engineered, the good ones to have a proper bevel angle to permit the fret to be mechanically "lifted" simply by the act of closing the nippers.

Thank you.

I'll try tapping it gently then.

If it doesn't go down or stay down, there must be excessive dirt or glue in the slot, we'll see.

Thanks again for the advice!