Are these spider cracks or simply cracking due to a heavy application of bottom paint? In either case, I presume this means I have some sanding and repainting to do. Should I be more concerned about this?

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could be do to a number of things, but yes that could and probably is the cause. and yes you do, I would soda blast it off and start with a clean surface.

How old of a boat are those pics of and what part of the boat is the pics of?

the boat is a 1969, scattered throughout the hull. Also noticeable on the forward deck. Those on the bottom are noticeably deeper than those on top (seems to coincide with paint thickness. The pictured area is on the left side of the hull, it runs from the back of the boat about 2/3's of the way to the front.

That doesn't look like bottom paint. Looks like old dry gel coat syndrome you just buy the boat? I can't imagine that they just appeared.

I bought the boat about 4 years ago. There had been some very minor cracking previously, but nothing like there is now. The sander has come out and i'm taking this stuff off. Per recommendation this will then get a few coats of Pettit Protect and will be followed by a few coats of Pettit Vivid to paint it white. I'm not thrilled to be doing this, but I guess it has to be done (I just wish it would quit raining so I can power through this).

I too would say that it looks like old boat dry gel coat syndrome . If it's on the deck also then it doesn't have anything to do with thick paint on the bottom . The last time I saw anything similar to that was on the cockpit sole of a boat that was old and had been left uncovered for years . The spiderwebbing cracks in gelcoat on hull bottoms that I've seen were all flex/stress related and didn't really resemble what you've posted .
Sometimes you get lucky and that's as far as it goes . Other times it starts to break away from the hull and drop off exposing the resin and glass of the first outer layer .
I guess it all comes down to how much time and money do you want to spend ?

BTW the Petit Vivid white is very good if you haven't already used it .

I believe before this boat was restored it had sat unattended for a number of years. I do not have any of these cracks on the deck. As I sand this down are there things I should be looking for? Once it stops raining I should be able to get to the glass in at least one area and see what is going on & i'll take pics and post back here.

WOW - 1 gal of Pettit protect, 1 gal of Vivid @ westmarine = $410...for 2 gallons of paint.

Sanding is surfacing what look to be a yellow and red primer coats, a super thick hard/white layer(s) that could be white gel and then what looks to be a couple layers of white poly on top of that....there is a lot of paint to come off. I looked to rent a soda blaster but no luck, so this is coming off by hand.

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I have about 6 hrs of sanding into this, I easily have another 18 to go.