Question: I made the decision over the winter to totally repaint the drive leg on my boat. The paint had been chipping over the last several years and there were small areas where slight corrosion was beginning to appear. So in the photo you see I have been sanding the old paint off and am down to bare aluminum in many places. I’m a little worried about what primer to use to ensure that the new paint job lasts a long time. This has been a lot of work in the prep so far, and I don’t want to have to repeat this in two years. Any suggestions on where to go from here?
Answer: This process is an easy place to make mistakes. In fact most people and even professional shops don’t do what’s really needed in this case to ensure that the paint job will really last as it should.
You don’t say if this inboard-outboard drive has been used in a salt-water environment or not, but I’ll assume it has been. This means that the drive has spent part of its life in what are known as soluble salts. Even in fresh-water environments soluble salt presence is possible, although not as prevalent. The bottom line here is that in spite of all your hard work -- and from your photo you have really been sanding vigorously on that drive unit -- soluble salts are still going to be present on the surface of the metal. In fact, because you have been sanding, you have actually driven into the surface of the metal some of these totally invisible particles.
Soluble salt particles under a paint film will form a microscopic blister that will eventually grow to a point where it ruptures and provides a microscopic access point for water ingress to the surface of the metal you’re trying to protect from corrosion.
So, the bottom line here is that these salts need to be removed before any paint primer is applied to all your hard work. Power washing or solvent washing of the drive will not get rid of the salts. Power washing could work, but the pressures required are extreme and well beyond anything you’ll find in a typical boatyard. We’re talking 20-30,000 psi pressures needed! Not the typical 1500-2000 psi pressures most boatyard power washers have available. The very high-pressure washers are quite common in shipyards, but not boatyards.
So alternatively, a salt-neutralizing agent is the best and safest solution. The product I’m familiar with is called Chlor-Rid.
Once you’ve washed the drive with the Chlor-Rid, use either Interlux Primocon or Pettit Aluma Protect primer to prep your drive for a final color coat. You will notice that neither Interlux or Pettit will mention anything about soluble salts in their product instructions. I encourage you to do a little research on this Chlor-Rid page to learn more about soluble salts and corrosion. Those of us who work on the corrosion analysis side of the marine industry have witnessed the impact of soluble salts on paint systems for years, so don’t let the fact that the paint guys don’t mention this lull you into ignoring it.
For an overview of painting projects on all exterior surfaces — topsides, deck, and bottom — read How to Paint a Boat.