What is involved in fret leveling?

in very simplistic terms:

first, neck is made as straight as possible (strings off, of course)...

next, a very straight block with an abrasive (such as 320 grit wet/dry paper) is used to grind the frets evenly (just enough so that the lowest fret is JUST touched)...

next, the frets are re-crowned using crowning files (there's another "dirtier" method which I won't get into). Fret tops should be round, not flat, definitely not sharp.

next the ends are tidied up using files...

finally, the frets are polished by some method (some folks use jeweller's rouge on a buff wheel)...

bass is restrung...now with good level frets, the action can be brought lower

 

Pilbara's explanation is a good outline of a dressing, otherwise known as a Level, Crown, and Polish.

Getting the neck straight is key. More importantly, getting the fingerboard straight is even more important. A good tech will use a notched straight edge to read the fingerboard surface rather than the frets. There is some debate on the use of jigs when straightening necks. There is a school of thought that the neck should be straightened under tension in the playing position and jigged so that this condition can be replicated at the bench when the work is being performed.

Once the neck is straight the fingerboard is masked off with auto detailer's or painters tape. The frets are then blued with machinists dye or a felt tip marker so that leveling progress can be seen. Files or abrasives are then employed to level the frets. It is critical that the file or abrasive backer block is dead flat. It is also important that almost no pressure is applied to the chosen grinder because it is very easy to bend the neck out register. This is a game played in thousandths of an inch. Make that less than a thousandth of an inch if ridiculously low action is desired. This activity is continued until the lowest fret is just touched.

Sometimes there are ruts in the frets. It is usually quicker to pull these frets, replace them, and begin the fret dress. This is known as a partial refret.

Crowning is accomplished a number of different ways. Most techs will used one or another of files with round, fret shaped profiles. These files are available as a traditional toothed file or with diamond abrasives. Others will use a triangular file and sweep it over the crown of the frets until only the smallest line can be seen. There are other methods.

The fret ends are addressed next. A small file is used to redefine the bevel and to remove any little burrs that are generated during this process and the rest of the work. Just because they can't be seen doesn't mean they're not there. The ends will be polished in the next step.

Polishing starts after the frets are leveled. Some will use an extra fine file to remove the smallest scratches before fine polishing. Others will move quickly to fine abrasives. The down and dirty method is to use some 600 grit paper and follow with 0000 steel wool. Some will use micro mesh abrasives starting at 2400 grit and moving through 12,000 grit. Others will take the neck to the buffing wheels. Depends on available technology and how much you like cleaning the shop.


When? When there are visible marks on the frets. The idea is that frets can be dressed many times before they need to be replaced. As mentioned before, if there are ruts in the frets it is probably moved beyond a simple fret dressing.

 

Another thing to watch for is the situation where you just have one or two high frets, or a few high fret-ends. In that situation you can usually just level/reseat the offenders and see a big improvement. I'll take a few straightedges, including a StewMac fret-rocker, and go over the whole neck looking for high and low frets. I've seen a lot more basses that had a high spot or a popped fret end causing a problem than basses that needed a full level-dress-polish.

The other reason for doing a high-fret inspection and fix as a first step is that it is a good prep step for a l-d-p.

In general, a good fret job and setup is the best way to make a bass play a lot better. How much work that involves depends a lot on the bass in question.