I am making a guitar and have slotted the neck into the body but there are some large gaps between the neck and body. Is there any foolproof way to disguise these gaps. I have been thinking of mixing glue with sawdust to fill the gaps but I am wanting more ideas.
There is no foolproof way. Here's a rundown of some options:
Use slivers/wedges of real wood glued and pushed into crevices. This offers the most strength and tends to look best, but is probably the hardest to achieve.
Fill the gap with a gap-filling glue, like epoxy. Epoxy will offer some strength to this mistake. Even if you fill the epoxy with sawdust or a die, it will tend to look like a filled gap.
The old wood glue and sawdust trick. Easy, but will stand out similar to epoxy. Regular wood glue will not strengthen the area like epoxy.
Real Wood filler. Has the potential to look better than the glue options, but usually not by much. It will also not strengthen the area.