You can also add an additive called flow-tex or flow-rite as I remember which help the paint stay flexible for longer.
Stik is right though always a two-pack if poss as the cellulose based paints do go brittle quickly. They do however have benefits of making cellulose based doping repairs easier.
By the way also if your paint is losing its gloss you can always overspray it with a cellulose based laquer which is 90% solvent so dries off to give a great finish again and weighs very little.
Acrylic laquers are good too but usually need a catalyst to make them go off hard - but they look like glass (really sharp).
Heavy though.........
Enjoy
Arc