Spider cracks are non-structural damage to the gel-coat of a boat. This damage is often caused by slight flexing of the hull. These hairline fractures also occur in boats of the mid 1970's vintage which often had thicker gel-coat layers than newer boats. This thicker gel-coat cannot stand ANY flexing without showing cracks.
There are several methods to make repairs.
The quickest and easiest way to remove the appearance of spider cracking is to apply a quality paint job to the boat (or at least the damaged area). This method is best used above the waterline.
Start by deciding which areas of the boat need attention. It may be possible to paint only a portion of the boat. For example, don't paint the entire boat if the deck is the only area that needs repair!
Next, remove all hardware and accessories that you can from the area to be painted. It is far better to remove a fixture than have to sand and paint AROUND it!
Third, remove all wax and grease from the area to be repaired with an acetone wash.
Next, sand the area to be painted with 100 grit sandpaper. (An orbital or DA sander will work fine here.)
Apply a quality primer (like Interlux Pre-Kote) per manufacturer's directions and let dry.
Sand with 120 grit, wipe down, and apply another coat of primer if required.
Sand with 220 grit, wipe down and apply top coat. I use Interlux Brightside. Apply with a foam roller and then tip with a quality brush or foam pad.
Allow to dry thoroughly between coats and sand with 220 grit. Wipe it down and repeat. Usually, two coats is plenty.
The other repair for spider cracks involves a little more work!
Start by wiping down the area with acetone and sanding the damaged gel-coat down to the fiberglass. Try not to remove any of the fiberglass resin or cloth!
Next wipe down the area with acetone again!
Color match gel-coat with your hull color. (This will probably take several tries!)
Catalyze the gel-coat and brush onto the exposed fiberglass area. Lay it on thick, but not so thick that it sags.
Wait for several hours and then lightly spray PVA (Polyvinyl Alcohol) The PVA serves as an air barrier, allowing the gel-coat to fully cure.
Allow the gel-coat to cure for a day or two, then wash the PVA off with soapy water.
Sand the area with 1000 grit paper on a DA sander, working up through 1500 grit to 2000 grit. The 2000 grit should produce a near-mirror shine!
Wipe the area down with acetone.
Buff the area and apply a quality wax to protect the finish.