Whether it runs along the outside of your house or punctuates the ceiling in your basement, an unpainted galvanized pipe is seldom attractive. Galvanizing protects steel drainage, duct and water pipes -- as well as metal gutters -- from corrosion, but it leaves an insipid grayish patina that actually resists certain types of paint. The keys to getting paint to stick include properly cleaning off the surface residue and avoiding oil-based paint.
Without a protective coating, steel -- which is mostly iron -- combines with oxygen and moisture in the atmosphere to produce rust -- a process known as corrosion. To prevent this, pipe manufacturers galvanize steel pipes by coating them with a thick layer of zinc. They either dip the pipes in a vat of the molten metal or use electroplating techniques. Before shipping the pipes, manufacturers often coat the galvanized metal with oil to retard reaction of zinc with the atmosphere. When this oil coating wears off, the reaction of zinc with oxygen produces a fine whitish film that changes the color of the metal from gray to an even less appealing whitish-gray.
If you've just had new ductwork or galvanized drain pipes installed, there is a good chance that the metal still has a layer of oil on it; before you paint the metal, you have to remove the oil. A solvent such as naphtha or lacquer thinner is best for this; moisten a rag with either solvent and thoroughly wipe down the area you intend to paint. Use either solvent with care: Both are flammable, and both give off dangerously noxious fumes. Wear a respirator while working; keep the room well ventilated, and avoid open sources of flame, such as gas heaters or lighters.
If the pipes you want to paint have been in place for over a year, the oil has probably worn off, and the zinc has begun to react with air, which leaves a powdery, whitish residue. Clean off this residue with a strong soap solution; 1/2 cup of trisodium phosphate in a gallon of water will not only remove the powder, it will etch the metal underneath and make it more receptive to paint. If you're painting very old pipes, such as old water pipes in your basement, use a wire brush to physically remove any rust or powdery deposits you see.
Zinc reacts with the solvents and binders in alkyd and oil-based paints to produce a soapy film that causes the paint to peel and slip off, so avoid those types of paints unless you find one specifically designed for painting galvanized metal. A water-based direct-to-metal, or DTM, paint designed for marine applications is the most reliable choice, but any quality acrylic latex paint will adhere. To be on the safe side -- and ensure reliable colors -- prime the metal with a water-based primer before painting. Apply the paint with a brush, roller or handheld airless spray gun. You'll probably need two coats; allow the first to dry for two hours before applying the second.