Chips and scrapes in a carefully painted mast start out as a cosmetic issue. But as moisture intrudes and corrosion takes over the paint blisters and hard metal becomes powdery aluminum oxide. Left unattended, especially in damp, salty areas, such as in the bilge or underneath mast hardware, this corrosion process can destroy metal and impair the integrity of the spar.
Corrosion is often exacerbated by other alloys that are in direct contact with an aluminum spar. They create a galvanic interaction between the two different metals. The stainless steel hardware in the photo below is a good example of how oxidation and galvanic corrosion conspire to destroy aluminum. Notice the paint blistering around the perimeter. There is no sign of either a dielectric plastic gasket or bedding compound separating the two metals. Dissimilar metal corrosion takes place when one material is significantly more noble (less corrosion-prone) than the other. In this case, the 6061-T6 aluminum spar behaves like a sacrificial zinc you put on your hull, corroding due to its contact with the more noble stainless steel hardware.
Spot paint repairs can be done and are always a good idea as long as the remainder of the painted spar is in good condition. Otherwise it may be time to un-step and go through a complete strip, prime and paint process. The best approach is to start with a top down inspection tallying up the bad spots. Minor coating damage without any deep pitting can be considered cosmetic. But in areas where there’s extensive blistering and deep pitting, structural concerns may be warranted.
There are two common, and distinctly different levels of paint failure. In the first, there are random scrapes and corrosion spots on the spar. These are indicative of minor cosmetic corrosion and are fairly easy to spot repair by following the steps listed below.
The corrosion seen around the perimeter of the stainless steel fitting shown in the adjacent photo is a more serious problem. Before tackling the paint failure, the fitting needs to be removed by drilling out the pop rivet heads. Carefully clean away all aluminum oxide using white vinegar and a stainless steel wire brush. Inspect the underlying aluminum, noting any cracks, deep crevasses or extensive pitting around the pop rivet holes. If there’s any deformation of the spar wall itself, it’s time to have a rigger take a close look. This is especially true if the hardware handles tensile loading from standing or running rigging.
The prep/paint process begins with removing corrosion and returning the area to bright, shiny aluminum. Use the wire brush mentioned above to remove oxidation from pitted regions. Sand with 80 grit paper to achieve a smooth but “toothy” taper from the good paint to the damaged area. Wipe with an alcohol or acetone and a clean cotton cloth. The next step is to etch prime the surface and follow up with a two part epoxy primer or single part primer depending on what type of top coat is being used. Before top coating, sand with 120 and 220 grit paper.
Most painted masts are white and there are so many shades of the color that a perfect match is all but impossible. The best answer is to purchase the exact paint used the last time the spar was painted. If not, an “almost the same” color will look just fine and more importantly, corrosion will be halted dead in its tracks.