To be clear , the wear in the photo above is not "natural " . just made to look that way ./( sand paper , and instant coffee stain in appropriate areas ) ;-)
Right, it looks like unintentional ware, that’s the art
I do a lot of artwork for a Nature Centre. One thing I do use is the clear coat exterior Varithane to coat projects. The SPRAY version which is also water based. This can seal the paint and the box. Since you used a marker, I would not use anything with a thinner base in it that is for oils or laquers etc. Water based products will not run the paint or the pen marks. Always test something similar that you don't want to use or is scrap. I have found this process to work very well! I have even made play cameras with paints and insulation foams, and finished coated with this method. The new exterior products are plastics, and they hold up much better than they used to do! If kid proof, it should also be instrument proof. I have also built , carved and created many different instruments, and used the spray methods for some of the finishes. On wood boxes, don't forget to try out the old fashioned gun stock oil! That simply is the best oil out there and instruments can have anything from soft satin to high polish look when using gun stock oil. Hope this helps:) Cheers!
I sprayed some water-based Varathane on this box. Worked pretty well. I’m sure it will eventually get some natural wear, assuming the guitar ends up being a good one...it will just take a little more time.
During the build I put blue tape on the areas I’ll be cutting and mask the rest of the box with some heavy weight paper to protect it. For finish I use Minwax fast drying Polyurethane on most of my cigar boxes. It goes well over everything. Spray the first coat light, then recoat within an hour and walk away. Hit it lightly with 320 the next day and spray a third coat nice and thick then walk away. Let it cure for a couple days and repeat the process.
That does work great. But, if you can only spray a couple of times, you can also brush on the rest of the coats. Gives it an artists look too as well as going on a bit thicker, but be careful not to create runs. . Also, you can also use gun stock oil to do the instruments:) That is an original finish for many instruments even today. I have had much success with using that process too:) However with cardboard projects, not wood, the poly does tend to allow for more flex. Just my experiances...:)