Why are two part paints better than one part paints?

Alkyd enamels, and modifications of alkyd enamel technology, have been the work horse products in the marine industry for approximately fifty years. Generally, they dry reasonably fast, are easy to apply and have very good weathering characteristics. More alkyd enamels are still used than any other enamel type. Their properties are well known to all experienced yachtsmen. They have been the backbone of the Interlux Product Line, and are respected by their tradenames – Interlux Toplac, Interlux Brightside Polyurethane, and Interlux Yacht Enamel. Today the fact that excellent finishes can be obtained from them with a minimum amount of effort is probably their biggest selling point. Two-part aliphatic polyurethanes, in comparison to alkyds, have superior properties, specifically: higher gloss and color retention, longevity, chemical and abrasion resistance and overall film integrity.