I'm a (perpetual beginner) sitar player, and haven't played guitar for many decades since playing all 3 chords in the garage band at the hippy farm. One recent day, there was a forlorn classical nylon-string guitar lying on top of a pile of trash bags with its headstock attached to its neck by only the strings. It had a "Karl Hauser" decal on the top, "Ideal Musical Merchandising Corp" label inside, and "made in W Germany" sticker on the back of the headstock. "Karl Hauser" was a house brand used by Ideal, presumably chosen for similarity to but with no connection to the luthier Hermann Hauser. I thought it was a junky cheap mail-order type thing, but I brought it home anyway, glued the headstock back on, bought new strings. Gradually realized it is actually quite nice. Beautiful wood, uncracked spruce top, book-matched maple back, maple sides, good tuners, and a truss rod(!). Probably at least 60 years old judging by the tuners which have non-modern spacing and smaller diameter all-metal string take-up axles. It sounds good. It has a few issues: heel separating from body; still somewhat high action after lowering saddle as much as possible. With nothing to lose, and so as to have an instrument that I can play like a sarod, I used it as a guinea pig for DIY defretting practice/experience.
I tried 4 different ways of filling the fret slots: starting from the sound hole:
1. A friend suggested baking soda and wood glue. Maybe I misheard, should have been baking powder? Anyway I didn't get the mix right, it set very hard but I had to top it with epoxy.
2. Elmers wood fill. Worked well.
3. Clear epoxy. Set dark, giving subtle but desirable visible line.
4. I had some 1/64" birch ply on hand. It seemed to work well and look OK, so I did the rest of the neck this way.
Best is epoxy in that wafter sanding, with eyes closed it is impossible to detect with fingertip or nail.
If I do another fret-refill, it will be either with epoxy, or dark hardwood.
The project is a success! Ive ordered some T-I Classic N flat wound strings.
My 2nd project is stainless steel over an existing fretboard. I plan to fill the slots with elmers wood filler, because it will leave visual cues to fret positions on the side of the neck under the metal.