What happens to fiberglass when it gets old?

Answer 1

Generally not much.

The glass fiber is inert to most normal things - water, salt, UV radiation, oils, fuels, moderate acids and alkalies.

Different resins are used to bond the glass fibers together, but they usually have good resistance to all of the above except UV radiation. This is why fiberglass structures usually have a gellcoat or painted surface to protect it from the sunshine.

With long-term sun exposure, gellcoats and paints will fade or chalk. This is an appearance issue and doesn't harm or change the fiberglass underneath.

The two most common resins used with fiberglass are polyester resins and true epoxies. Fiberglass made with polyester resins can form blisters when it is in continuous contact with water for long periods of time (years). Osmosis causes the water to soak in between the layers of fiberglass. Since polyester resin is cheaper than epoxy, it is commonly used in consumer products like boats. It is common for older boats that have been kept in the water for years to develop blisters below the waterline. The blisters don't really seem to cause any problems, but boat owners hate them and will sometimes pay thousands of dollars to have the outer layer of the hull striped off and replaced. Epoxy resins don't generally have this problem.