Wetting the insboard well before applying the patch helps a lot.
Simplest patch - Cut out or dig out the worst area and replace it with a patch of 1/2" inswool and satanite (see below).
A bit more work, but easy to do - Use rammable refractory to patch insboard. I put a whole new bottom in my NC Forge this way. It allowed me to curve the bottom up onto the sides a bit to get a tad of swirl, too.
Another option is to apply a 1/2" thick inswool lining and satanite over all the insboard. The 1/2" stuff is pretty cheap, and a Diamondback could be relined for less than $50.
Here is a source:
https://www.ebay.com/itm/Kaowool-1-...ulation-Blanket-8-Muffler-Stove-/191984962522
A third option is to place 1/2" insboard over the current lining. it runs around $15 a sq.ft. from HTT&R.
A final tip is a trick I use to hold refractory on covers for vertical forges:
Run short coarse thread drywall screws into the insboard through the wool to anchor it before covering with satanite.
On a round metal lid to a forge, I tack welded the screw heads to the steel top with the points sticking out, and pressed the wool down on them. I worked a thin slurry of satanite in first, then finished with the normal 1/4" to 3/8" coating.
Drywall screws come as short as 1/2", BTW.
