Rib repair using spruce diamonds.

I do it this way to reinforce the cracks only, without excessive stiffening of the rib, nor restricting its ability to move with changes in humidity.
I generally remove all previous work if it is failing and the cracks are open. I clean the inside of the rib with cold then hot water and scrape all old glue off. I clean dirt out of cracks and fill any cracks that will not close with moderate pressure. I generally press wet ribs with thin acrylic on the outside surface to level the ribs first, rubbing in some hide glue and clamping the cracks closed if possible. I use rubber or  film-covered foam as a clamping surface on the inside of the rib. I leave overnight. When dry, I scrape the indie back level and then work quickly to reinforce the insides.

I use the diamond cleats to align the cracks and restore the shape of the rib curve, often distorted with age and rib damage and previous repairs.

The concept is to fit a cleat to an adjacent rib section with correct curvature. Then, moisten the rib and press the cracked area onto the shaped cleat with glue. The rib, being more flexible, will conform the the shape of the cleat. When the glue is dry and the shape restored, the cleats are neatly trimmed and thinned to their final shape.

I cut rib cleats from nice quartered spruce, I make them square in various sizes, with the grain parallel to one side. I generally make them around 6mm thick, thicker for steeper curves. The finished cleats should mean that the grain runs diagonal to the rib.

Layout of the cleats should follow the size and direction of the cracks. The diamonds should be aligned with the points precisely along the crack itself, and generally be spaced an equal distance apart. Cracks that are close to one another should be done with cleats that cover one crack only, so smaller, and the cleats for adjacent cracks should overlap neatly in their layout so as not to create new weak points. I generally layout and number the cleats for each crack before fitting. I mark "north" on each so i know which way they point. I also mark a pencil cross on the glue-side so i know when the fitting sanding has been done evenly.

The cleats are shaped against a wide sanding block covered with good-quality non-clog 80 grit sandpaper. The block has a variety of possible curves on it.

I find a spot on the block that matches the correct curvature of the rib, then sand ACROSS the block carefully until all traces of the pencil cross are removed. This should align accurately with an undamaged part of the rib adjacent to the crack. 

I apply hide glue and clamp each cleat to the correct spot, using a three-pointed caul, or several clamps, if the rib needs coaxing back into shape. Again, using thin acrylic of polycarbonate on the outside to keep the crack edges aligned.

When the glue and ribs are dry (overnight) I remove clamps and cauls, then trim the diamonds into neat pyramid shapes with a small plane or sharp chisel. I clean up, scrape and wash off any surplus glue. I finish with a small oval MDF sanding block faced with 240 grit to get everything looking smart. 

I use thin artist-grade raw linen to provide stiffness and support for delicate areas, also to support uncleated ribs near extensively repaired areas. This is to ensure there are no new weaknesses caused by repairs. I cut the linen so that the warp and weft is glued diagonally to the rib edge. This allows the fabric to strech slightly with humidity changes. I shrink the linen with boiling water before use, and cut the edges with pinking shears to leave a neat border. After gluing with hide glue, I lightly sand any sharp raised edges and seal with another thin coat of hide glue.