Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

September, 2002


Q. What would a common procedure be to resurface a corrosion pitted marine motor outdrive (mercury-aluminum outdrive)? I have heard that 2 part paints and epoxies have incredible adhesive properties as well as corrosion resistance, but what would be recommended in this application?

A. An epoxy primer followed by a polyurethane topcoat is still my number one choice, but the real issue is cleaning up the pits in the motor outdrive. Once the pits have been thoroughly cleaned, your friend will need to immediately apply the epoxy primer. By immediate, I mean within a few hours and not the following day. One method to clean the pits and prepare the surface is by abrasive blasting with glass beads. But your friend will probably need to take the drive to job shop to have this done.

In the industrial world the drive would probably be pretreated with a chromate conversion coating, but in all likelihood your friend won't have access to this process, unless he/she is willing to take the drive to a job shop that has the capabilities of pretreating aluminum. (Many job shops can blast clean the surface with glass beads and pretreat in a conversion coating).

If your friend does not want to take the drive to a job shop, another pretreatment coating is a "wash primer" which is a very low viscosity, high VOC organic primer that contains phosphoric acid, a vinyl butyral resin, corrosion inhibitor and solvent. I'm hesitant to suggest the application of a "wash primer", because if it is not properly applied blistering of the coating system can result. Therefore, I'm left with the recommendation that he/she apply the epoxy primer directly to the clean surface. Remember, the degree of cleanliness will determine the success of the recoated surface.

Unfortunately there are no guarantees, because so much depends on the substrate and the surface preparation. Good luck

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