Plastic gutters have several advantages over aluminium and wood gutters such as easy installation and little maintenance. Plastic doesn't rust like aluminium and it is more resistant to water damage than wood. Plastic gutters are also less expensive than aluminium or wood gutters. The downside is that the plastic, which comes with a baked on finish, eventually fades and ages when exposed to the ultraviolet rays of the sun and other weather conditions. The temptation is to replace them with new gutters; however, they can also be brought back to life with a fresh paint job.
Remove the downspouts. Downspouts are much easier to work on when they are not attached to the house. Painting them in a horizontal position is much easier than moving up and down on a ladder and worrying about getting paint on the siding. Lay the downspouts on a couple of sawhorses.
Wash the gutters and the downspouts with warm water and a detergent. Fill a bucket with a mixture of water and TSP(Trisodium Phosphate). TSP is a very strong cleaner used to wash surfaces prior to painting. Mix one cup of TSP with 3 quarts of warm water in a bucket. Use a sponge to remove all the grime, dirt, and other debris. Protect your hands with rubber or latex gloves. Allow the gutters to thoroughly dry.
Paint the gutters and downspouts with an oil-based glossy paint. Oil-based paint works very well on plastic and does not require a primer. Apply the paint with a brush and a small 4 to 5 inch roller. The roller makes the job go much faster and produces a smooth finish. Paint the edges with a brush and the majority of the gutter and downspout with the roller. Inspect your work as you go for runs and drips. Pay particular attention to paint that may drip on the siding. Wipe off any drips on the siding with a rag and paint thinner. Allow the gutters and downspouts to dry for 24 hours and then apply a second coat.
Reattach the downspouts.