We have an existing thread on all project basses below
@jonathanhaynes43 might be your man to help with this.
I’m pretty confident using wood stain etc but have never used leather dye.
What you need is for there to be no residue coming off on your hands / towel because when you use a sealer it will fail to bond properly.
Example if you have a table covered in dust and try and stick a piece of tape to the table it won’t do it. The tape will bond to the dust and pull away. Same principle.
I don’t know what product you can use to cleanup leather dye however the first thing I’d do is find some rags that you can throw away and hand buff the body. Rub it really hard until no more stain is coming off on the rag. If it’s still coming off then maybe something like naptha to give it a clean down? The problem is that it might lift / smear the dye. Then use a sealer that will work with the dye. It may depend if the dye is water or oil based.
Spraying on a sealer from a can will get you the best results. Watch this video to learn more than you’ll ever need to know about spraying from a can. I did a bass project last year and used this video and it came out great.