I have been using Rust-Oleum rattle can primer for the early parts of the build. It gets the job done but I have not been fully satisfied. It feels a bit muddy and the bonding with aluminum leaves something to be desired.
Mathieu at EAA Chapter 20, who is building his own RV-9, mentioned that he etches the aluminum before priming. He scrubs the surface with red Scotch-Brite using EkoEtch, then applies more EkoEtch as he works so it does not dry out on larger parts. Finally he rinses it with water and lets it dry for about an hour before priming. As per him, this significantly improves how well the primer bonds to the aluminum. That got me thinking more seriously about the whole priming process.
The Constraint: Non-Toxic
An important condition for me is that whatever primer I use must not be toxic. I am working in my garage in a closed residential space. I cannot be spraying solvent-based paints and breathing fumes, and I do not want to be that neighbor either. This ruled out most of the traditional epoxy primers used in aircraft building.
After some reading, Stewart Systems EkoPoxy kept coming up. It is a water-based epoxy primer, non-toxic and easy to clean up, and well regarded in the homebuilder community. The water-based formulation is an added benefit for working in a residential garage.
Getting Advice and Getting Started
I talked with Thomas, the technical counselor at EAA Chapter 20, and he suggested getting a smaller quantity first and experimenting before committing to it for the full build. Good advice. So I ordered a small quantity of the EkoPoxy primer along with the EkoEtch.
Stewart Systems EkoPoxy primer and EkoEtch, ready to experiment.
First Application
Today I went to San Carlos Airport and, with the help of Thomas and Mathieu, tried it out on a scrap part from the laser-cut parts pile.
Mixing the EkoPoxy two-part primer before application.
▶ Watch: Scuffing aluminum with red Scotch-Brite and EkoEtch (YouTube Short)
EkoPoxy mixed and ready in the cup before spraying.
The Paint Gun Problem
The gun we were using was a Harbor Freight HVLP, rated at around 6 CFM. Stewart Systems recommends a gun rated at 9 CFM HVLP or higher for proper atomization. That explains some of the inconsistency in the application.
I have ordered the 3M Accuspray Paint Spray Gun, which exceeds the 9 CFM requirement and is known for being easy to work with and clean, which is important for a water-based product.
The other piece of the puzzle is the air compressor. The Accuspray has a healthy appetite for air, so I will need a larger compressor to feed it. The plan is to have two compressors: the ultra-quiet one I currently use for everything else in the shop, and a larger one reserved for priming on weekends. The weekend-only schedule is deliberate. I need to be mindful of the neighbors.