The advice I have seen on this forum is to not worry too much about spider cracks...... Embrace the character of an older boat and go sailing. However, I have repainted two boats, both with spider cracks, and it worked great to simply sand them out. Just be careful to not sand into the gel coat. If you are not into painting the whole deck there are ideas for patching, but sanding has worked for me.
I am presently in the process of restoring a 1973 with a full compliment of spider cracks, chips, dings and whatnot. All are cosmetic and do not appear to be structural. I am currently putting the second coat of finish paint on the hull before flipping to start the deck. My approach was as follows:
One half of a container of Comet cleanser to scour the hull;
Major dings and chips were addressed with Marine-Tex (in thin layers to alleviate excess sanding)
Sand entire hull with 100 grit;
Applied System Three Quick Fair and sand with 120 grit;
Find more areas to fair and repeat above step;
Roll and tip Rustoleum Marine Coatings Primer
Find more areas to fair and sand for final time;
Light sand hull 150 grit;
Apply second coat of primer;
Wet sand hull 220 grit;
Apply first coat of Rustoleum Marine Coatings Top Side White;
I will wet sand with 400 grit today and then another coat of paint.
When dry, flip and start again.
I have thinned all my paint and left plenty of drying time between coats and so far not to bad. Spider cracks are currently not visible and I will report back to inform anyone interested of how the operation held up. After reading over 75 pages on this Forum I have consumed some interesting information. Am I a Body Man? No! And I was not going to spend the intensive amount of time required to grind out all of the spider cracks and apply new gel-coat on a $40/40 year old craft.
The System Three product, while not inexpensive ($48 per quart & a half) was much easier to install AND sand compared to the Marine-Tex which is just as expensive, pound-for-pound.
All of the above is only what I have done and is in no fashion designed as the proper method for approved repairs.