MIMF

I recently picked up this guitar that is quite a bit nicer than what I normally get to work on. I'm pretty sure I can do a creditable job repairing the back/side, but I'm a little nervous about repairing the soundboard crack unobtrusively, not having done this before. It's has been sitting in the basement humidifying for a month, but the crack doesn't seem to be getting any smaller.
I imagine it will be necessary to fit a sliver of wood into this crack. My plan right now is to get a couple of cheap soundboard blanks
and practice a bit first. It may be a while until I get to it. Would it make sense to glue a couple of cleats on the underside of the crack
to stabilize it until I fill the crack? Any advice would be appreciated. Thanks.

That crack doesn't look so bad to me. Does light shine through it? If not, I would just humidify it a bit more, and then clean it out, glue it and cleat it inside.

The crack is a bit over 1/16" wide at the bridge, narrowing down toward the bottom. Getting it to close on it's
own would be best, so I'll leave it for a while longer then see. I'm probably just worrying overmuch. The
repair itself isn't that difficult, but if it really stands out it's going to bug me every time I pick up the guitar.
I am in no way a professional at this sort of thing, but I try to make repairs that I do as unobtrusive or as attractive as I can.
Thanks.

A crack along the grain like this is usually not a structural issue, but more of a cosmetic issue. It really shouldn't hurt the use of the instrument as it is, and just remember that it is much easier to make it all look worse than to make it better. If the crack will not close with re-humidifying then I would likely leave it as it is. If it causes rattles and buzzes then it should be glued and cleated (hot hide glue).

Installing and finishing a splint is a daunting task. It is simple enough to comprehend but is very difficult to install and blend it with the original wood and finish. Sometimes it works, but it is a tedious task to make it look good.

Michael Lewis wrote:A crack along the grain like this is usually not a structural issue, but more of a cosmetic issue. It really shouldn't hurt the use of the instrument as it is

That's a new thought to me. I figured just about any crack was a ticking timebomb and needed immediate attention. I have a guitar that has cracked exactly as the one pictured in the OP and I've been trying to make time to get around to taking care of it. I guess I can relax a bit.

Usually a crack can be closed with putting humidity into the box, but sometimes the wood has undergone some change and the crack will not stay closed. I just happen to have two guitars in my shop with this issue. Both were exposed to extreme dryness and or heat. Once the cracks were nicely closed and glued I let them hang in my shop with RH 40%, and within a week they opened again.

I have had guitars from Reno NV (very dry and hot in the summer) with tops sunken from the excessive loss of moisture content. If you stretch a thread across the top just in front of the bridge (on Martin guitars) the thread should just touch all across from edge to edge. When a guitar is very dry there will be a gap between the thread and the top, sometimes as much as 1/8" (3 or 4mm) or more. The guitars can be re hydrated and this helps but often they will not come all the way back to where the thread will contact the top by the bridge.