When I worked on the side crack on the Gibson mandolin, at some point I noticed this crack in the top. I just didn't pay much attention at first, until I realized it was the center seam and it was a crack that needed attention.
I'm actually holding it open a bit in this picture. It wasn't quite that bad looking and open in reality (maybe that's why I sort of missed it even though it was plainly visible...).
Anyway, I set about fixing it. Top cracks are generally the most straightforward body repairs on an acoustic instrument, since they're the most accessible.
I discovered my cam clamps were too big to get into the soundhole, so I improvised with this cast iron c-clamp. I just stuck some cork on the bottom with rubber bands so it wouldn't damage the inside bracing of the mandolin.
Then I reached in from the end pin hole with some long tweezers with an angled end and pushed up on the top. Then brush some hide glue on, while working the crack up and down gently.
And repeat from the soundhole end - with fingers inside the mandolin.
What you see above is just a test with some glue. I wiped that off with a warm, damp cloth before the actual operation. When I did the actual repair, I was able to get the crack nicely opened up and glue worked into the seam.
One of the things I really like about hide glue is that it's so easy to get excess off, even if it dries. In this instance, I put a pretty liberal amount on, then wiped the excess off quickly before I clamped it up.
I won't bother cleating this from the inside. The crack will be pretty tightly closed up, and the hide glue tends to pull the crack together as it dries, making for a strong repair.
I only cleat when absolutely necessary - meaning when a joint needs support. This crack will be solid.
My guess is the seam has been open for quite some time, so I'm happy with the way it closed back up.
Love that sunburst! Much more subtle and natural than Gibson's 1950s sunbursts, I think.