Priming Brass and Aluminum
I have discovered that I have not been treating the problem of priming the brass and aluminum parts of my whirligigs with enough attention.
Priming, brass and aluminum metals present special problems. I discovered the problem of getting paint to adhere to these metals, when strong winds blew over a few of my whirligigs. I noticed the paint cracked off where the metal part was bent. (Fig. 1) When I tried to sand the painted areas around the bare metal, more paint chipped off easily. It was clearly a problem of the primer not adhering to the bare metal. Only the top sign paint coat was holding the whole painted surface together.
fig. 1
To solve the problem I first called some local paint stores. Their advice was not very helpful. Next, I searched the internet. Paint manufacturing websites surprisingly had no specific information. A more general internet search lead me to sites of model train aficionados. Many model trains are cast brass. After sifting through a lot of conflicting information, I set up a simple experiment to determine what would work.
Experiment
- I cut four metal samples to about 1“x 2“, two each of brass and aluminum, and drilled a small hole in each so that I could hang for spray painting.
- I cleaned each sample, including the edges, with #1 steel wool.
- Immediately, I put one pair of samples in the dishwasher and the second pair in a solution of TSP, trisodium phosphate. I ran the dishwasher through a regular cycle with “Cascade with extra bleach action.” The TSP solution was 1 cup per gallon. I left the samples in the TSP solution for the same length of time as the dishwasher ran.
- I rinsed and dried both sample sets.
- Immediately, after drying them, I put both sample sets in a 5% solution of white vinegar (how it comes out of the bottle) for five minutes swishing the samples a few times.
- I rinsed the samples in tap water and dried them with a paper towel.
- Immediately, I sprayed Rustoleum Clean Metal Primer, very light gray, (no product number) on all surfaces of the metal samples. Following the directions on the can, I sprayed a second coat 48 hours later.
- Again following the directions on the spray can, I waited at least 48 hours to test the paint adhesion on all the samples. Another internet search led me to approximate the standard adhesion test. With a box cutter, or X-acto knife, on one side of each sample I cut an “x,” on the other a grid of closely spaced lines (about 1/8” apart.) (Fig. 2 & fig.3) For the test I used cheap grade masking tape that is widely available. I applied the tape to the scored sample surface and burnished it down with my fingernail. I steadily and quickly pulled (not ripping) the tape off the sample holding the free end of the tape as nearly parallel to the sample surface as possible. (Fig. 4)
fig. 2
fig. 3
fig. 4
Results
There was no lifting of the primer coat whatever from any to the samples.
Discussion
The process outlined above will give a primer coat that does not flake or peel away from the brass or aluminum surface. The parts may be cleaned either in a dishwasher with Cascade or in a TSP solution. I emphasize that the process from initial cleaning to priming has to be done in one operation; otherwise, the metal surface will have time to oxidize. Oxidation of the metal surface, the union of metal atoms with atmospheric oxygen, happens quickly. The paint will not adhere to the oxidized metal surface. I suspect the reason for better adhesion to the cleaned surface is that the metal atoms very easily give up their electrons (Al to Al+++.) The freed electrons may form weak bonds with the molecules in the paint primer, helping the primer to adhere. When the metal is oxidized, the oxygen in the metal surface holds very firmly to the electrons, and the paint does not bond at all. This is only my guess.
