I finally decided to fix the spider cracks around the rudder on my boat. A previous owner had gently hit something with the rudder, just enough to crack the gel coat, but not enough to damage the fibreglass or bend the rudder.
Yesterday I ground out the cracks with a Dremel tool with a diamond tip bit.
I used about 1/2 oz of white with 2 small drops of black pigment. The MEK was added and mixed at 2%. The repair was covered with plastic to cure. The colour match is very close to the original sanded gel coat, but is lighter to the discoloured hull.
The first layer of gel coat went on last night. Working upside down on the bottom makes for a rough first coat. After sanding today I applied the second coat to fill a few of the low spots. Sanding from 100 grit to 220 grit for the first knock-down. Tomorrow I will use 220-1000 for the final. Following that I will buff it out. Of course, the sanding will come after I return home from a day on the water. Why waste a good morning sanding...