You can read for days on the internet about priming the aluminium parts of an airplane, so many opinions, dos and don’ts.
Next question is “do we need a chromate conversion coating”? Most used is the ALODINE 1201 product of Henkel:It does not add weight to the airplane, has very good corrosion protection and paint sticks very well to it. It gives the aluminium a golden color. The liquid Alodine is difficult to buy in Europe and is very expensive; about 50 Euros per liter! The powder Alodine (BONDERITE M-CR 1200S) is packed in packages of 10 KG, costs about 3oo Euros and will give about 1300 liter of Alodine immersion fluid. So that’s less expensive. To prepare the aluminium for the Alodine, another fluid is required like ALUMIPREP NO. 33 or BONDERITE C-IC 241 and it’s about the same price as the Alodine. After the preparation fluid and the Alodine immersion, rinsing with water is required; so 4 tanks are needed. The size of the tanks should at least be 150 cm long, 30-40 cm deep and 20 cm height, so the spars will fit in, or even bigger to Alodine the skins. These tanks are difficult to buy, it’s not a standard size; a custom made tank is possible but very expensive. Since these are hazardous products it’s prohibited, and not very good for the environment, to flush it down the drain, it has to be picked up by a hazardous waste disposal company, they charge about 2 Euros per KG. Since 4 tanks of minimal 60 liter are required it’s another 500 Euros for disposal. A rough estimate of the Alodine immersion is around 1500 Euros! Another remark is that the temperature should be around 25 degrees Celsius, in the winter this would mean partly heating the shop. Last but not least these are very toxic chemicals with risk of lung cancer! The Airplane Factory in South Africa used Alodine for the first factory built Slings but stopped using it. So it’s very expensive, a lot of extra work and very dangerous; in my opinion better not to use it for an amateur built airplane.
A water-based corrosion protection advised by Zenith Aircraft is the Cortec VpCI-373 green for priming and Cortec VcPI-440 for cleaning:It’s non-toxic and only available in 5 gallon containers. The price in the Netherlands for both products is about 1400 Euros and shelf life is only 12 months; quite expensive.
A lot of primers found on the internet are only availabe in the USA (or difficult to buy in the Netherlands) like: NAPA 7220, Sherwin William, SEM, Duplicolor, Akzo, Rustoleum.
When building the practice kits of Vans I used a rattle can 2K epoxy primer from Spraymax:It’s nice for doing small parts, no need to clean a spray gun. But doing the inside of the skins with this product requires a lot of rattle cans, which cost 15 Euros each.
Other builders in the Netherlands used a 2K epoxy primer of Epifanes:It’s only 25 Euros for 750 ml and the color is white. When the aluminium is prepared correctly it sticks quite well. Since the interior of the plane should become gray I prefer a gray primer.
Looking further there is a 2K epoxy primer of a German company called Mipa:During a phone call, with the technical department of Mipa, they did not advise a 2K epoxy primer because the layer of primer is too thick. They advise a 2K wash primer, fast drying (30 minutes), chromate free with good adhesion on non ferrous metals and recoatability with all kinds of paints in today’s modern industry:
This wash primer is used by airplane builders in Germany, Russia and South-Africa. It costs around 25 Euros for 1,5 liter.
The local paint store didn’t have the Mipa wash primer on stock and advised a similar product from a dutch company Kristal Coatings:This CR50 2K wash primer is self etching, fast drying, has good corrosion protection and has a gray color. The price is 45 Euros for 2 liter.
Conclusion based on my current knowledge is to go for the 2K wash primer from either Mipa or Kristal Coatings.
Update: I received info from The Airplane Factory; they are using a primer from Plascon called: “Epiwash Chromate free primer”. There is limited info on the internet and it seems a South African product.