How to Touch Up Paint and Corrosion on an Outboard or Sterndrive

• Scuffs and chips from sand, gravel or the trailer, with clean metal underneath: a straightforward touch-up.
• Bubbles or blisters in the paint, often starting on sharp edges below the waterline, with white powder on exposed metal: Mercury describes this as an early sign of galvanic corrosion. Fix the cause (next step) before you repaint, or it will come back.
• Deep pitting, metal that is eaten away, or corrosion around the prop shaft seal, water intakes or drive mounting points: have a dealer look before you paint over it.

Paint only slows corrosion; anodes stop it. Find every anode in your manual (some hide under the anti-ventilation plate or in the trim cylinder area). Mercury recommends replacing an anode when about half of it is gone, and warns that anodes that never wear may mean they are not working at all. On a sterndrive with a MerCathode system, Mercury says it should be tested once a year by a dealer. If you added stainless steel parts (a stainless prop, trim tabs, aftermarket steering), Mercury suggests having your corrosion protection rechecked. See the sacrificial anodes and sterndrive corrosion guides.

Choose a dry day with mild temperatures, within the range on the paint can, and no wind blowing overspray onto the boat, the truck or your neighbors. Put the boat on the trailer, trim the motor or drive down so the area is easy to reach, and lay plastic on the ground to catch dust and drips.

Remove the key and lanyard, wedge a block of wood between a prop blade and the anti-ventilation plate, and take the prop off. Keep all the washers and the nut in order. Wrap the prop shaft in a rag or plastic so no paint gets on the shaft or its seal.

Scrub the area with soap and fresh water and a nylon brush, then rinse. Remove marine growth with a plastic scraper. Wipe the area with wax and grease remover or naphtha on a clean rag. Interlux gives a handy test: when you rinse, clean metal lets water spread out in a sheet; if water beads up in drops, it is not fully degreased yet.

Cover with tape and paper:

• Anodes. Never paint them. Mercury says anodes must be in direct contact with the water to work; mask them or remove them while you paint.
• MerCathode parts on sterndrives (Mercury says not to paint or power-wash them).
• Water intake screens, the telltale outlet and any speedometer pickup holes. Paint can block them.
• Sensors and transducers mounted on or near the drive, such as a trim sender or a transom-mount depth transducer, plus their cables.
• Chrome trim cylinder rods, grease fittings, rubber seals and bellows, the exhaust outlets and the prop shaft.

Boating Magazine's full-repaint project does the same: tape off grease fittings, anodes and anything you could not remove, and wrap wires and trim rods.

Scrape off any paint that is lifting or flaking. Mercury's method: where the aluminum is pitted or blistered, sand the spot with 180 or 220 grit until you reach clean, sound metal, then feather the edges into the good paint around it with 320 grit by hand. Remove all the white powder; painting over corrosion traps it under the new paint. Use abrasives meant for aluminum: avoid steel wool and steel wire brushes, which can leave steel particles behind that rust and stain. Interlux also warns that any blasting must use aluminum-compatible grit, never copper slag (blasting is a pro job).

Scuff the bare metal and the surrounding paint about an inch beyond the damaged area with a medium abrasive pad, so the new paint can grip (Mercury's tip). Wipe again with wax and grease remover or naphtha and let it flash off, then lightly wipe off dust with a tack rag. Do not touch the area with bare hands from now on.

Bare aluminum starts to oxidize as soon as it is exposed, so prime the same day you sand (Interlux says within a few hours). Use the primer your engine maker sells or names for its gearcases; Mercury uses a light gray primer under its color coats. Follow the can for distance, coats and dry time. When the primer is dry, scuff it lightly with the abrasive pad to feather the edges, without sanding through to metal.

Two warnings about primers:

• Do not use anything that contains copper, and do not use copper-based bottom paint as a "primer". Copper next to aluminum in water causes serious corrosion.
• Old how-to articles often recommend zinc chromate primer. Chromate primers contain hexavalent chromium, which is highly toxic. Stick with the primer your engine maker or paint maker specifies today.

Spray the maker's color-matched paint in several thin, even coats rather than one heavy one. Mercury suggests holding the can 8 to 10 inches away and letting each coat set up 10 to 20 minutes before the next; your can's label may differ. Overlap slightly onto the old paint so the edges blend. Mercury also offers a leveler coat that can be applied before the last coat dries to smooth out sanding marks; it is optional.

If a scratch has not gone through the factory primer, you do not need to sand and spray. A factory-color paint pen fills it in neatly. Suzuki's owner FAQ puts the rule simply: inspect the motor often, touch up surface scratches with color-matched paint, and on deeper scratches where bare metal shows, put a quality metal primer on first. Mercury sells pens matched to its colors; other makers offer similar products.

Mercury says to wait at least 24 hours for the final coat to cure before the boat goes in the water; follow your can's label if it says longer. Then remove all the masking, check that every anode is bare and clean, and reinstall the prop with the correct hardware, tightened to the spec in your manual.

If the boat stays in the water and you need antifouling on the drive, use only a copper-free product made for aluminum outdrives and lower units, applied over the primer its maker specifies. When you bottom-paint the hull with a copper paint, keep it off the drive and the transom assembly: BoatUS recommends leaving an unpainted margin around a sterndrive (it mentions about an inch and a half) and says to use a copper-free, drive-specific paint on the drive itself. Mercury lists the wrong type or wrong application of antifouling among causes of corrosion that its warranty does not cover. See the bottom paint guide.

• Flush and rinse with fresh water after every saltwater trip.
• Mercury suggests a corrosion-guard spray on the gearcase, drive and exposed metal, keeping it off the anodes.
• The top of a sterndrive often turns dull from mineral deposits. Mercury's FAQ suggests a bathroom tub and tile cleaner, then a coat of wax.
• Check the paint every haul-out and touch up chips before they spread. Mercury recommends a yearly touch-up.