Put on rubber gloves and clean the plastic fence post with mineral spirits and a rag. Allow the mineral spirits to evaporate away from the fence post once all the dirt and grime is removed.
Spread urethane-epoxy resin over the damaged spot using a plastic putty knife to smooth and even it. Let dry for at least three hours or as long as the manufacturer suggests.
Sand over the patch on the plastic fence post using 320-grit sandpaper, making it less visible, smooth and flush with the rest of the fence post. Wipe down with a tack cloth thereafter.
Clean the plastic fence post with TSP and water, using a sponge. Rinse off with water once cleaned, then allow to air dry.