Acid Etching / Metal Conditioning to Paint Aluminium
Before primer and paint will adhere properly to a metal surface, it must be etched. Etching is a process that removes oxidation and microscopically roughens the surface. The accepted practice is to use a phosphoric acid etch. However, many commercial process use an alkaline etch on aluminum then follow up with an acid etch to de-smut the surface. The risk of using an alkaline etch is that the alkaline salts will be left behind causing corrosion sites to develop. In any case, the goal is to remove oxidation and roughen the surface.
Another conditioning step is often done on aircraft aluminum. This step is a chromate conversion coating often called Alodine, although this is a trade name. The conversion coating helps protect the aluminum from corrosion in the field, and it also helps with paint adhesion.
Acid etching is a fairly simple process. For aircraft aluminum, phosphoric acid is the normal chemical used. Phosphoric acid is fairly safe in the concentrations used for etching, although if any gets on your skin it will burn and should be rinsed off. By the way, phosphoric acid is what give Coke and Pepsi their acidic taste or bite.
You can purchase acid etch from various sources including Aircraft Spruce (Alumiprep), welding supply stores, and Home Depot and Lowes (a Jasco product that looks green). However, it is recommended to avoid the stuff from Jasco. It will work, but it does not list any surfactants in the ingredients list. Alumiprep and the stuff from welding supply stores contain surfactants, usually ethylene glycol and phosphates (typical ingredient in laundry detergent, at least it was typical). The surfactants are important to emulsify any remaining oil, dirt, and metal removed during etching process.
Although you can buy acid etch off-the-shelf, for doing an entire airplane you may need larger quantities. Local chemical supply stores will often sell you what you need - 75% phosphoric acid and ethylene glycol. Sometimes the minimum quantity is 5 gallons, which may be in excess of $300. Sell the left over (a lot) to your local aircraft mechanics. Alumiprep and other off-the-shelf etching compounds will also contain hydroflouric acid. While this will also act to etch aluminum, its probable main use is to act as a brightener. Because this acid is not as safe as phosphoric acid, if making your own brew it is not recommended you purchase this. The phosphoric acid and ethylene glycol will do just fine. Mix one part acid, one part glycol, and one part distilled water for a 30% etch solution. Add 2.3 parts water to 1 part 30% solution for a 10% solution. Alumiprep ships at 30%, and sometimes you may want 30% for the really tough stuff. 10% is sufficient for most work.
Don't try to etch everything at once. Pick a relatively small area and work that to completion. Spray or sponge on the etch, and lightly scrub with fine (green) or very fine (grey) Scotch Brite. It will take a little practice to realize when enough is enough, but the real test is when you rinse the etch off. The water should form a break-free surface if the metal is etched properly. Because acid is consumed during the etching, you will need to spray additional etch on occasionally as you scrub the surface.
Because etching exposes fresh aluminum with lots of microscopic surface area that is easily oxidized, it is best to immediately follow up with the next conditioning process.
Aluminum Conversion Coating
All aluminum parts should be conversion coated for corrosion protection and good paint adhesion.
The traditional conversion coat for aluminum is Alodine. Alodine is really just chromic acid. In powder form, it is an oxidizer and needs to be handled and stored carefully. Chromic acid uses the hexavalent form of chromium, which is known to be carcinogenic.
For this reason, safer trivalent forms have been developed. One such product is Aluminescent. This supposedly a drop-in replacement for Alodine and meets the same milspec. However, it is quite finicky. pH must be kept within a certain range using potassium hydroxide and sulfuric acid (both are nasty) and a supply of pH indicating strips, the temperature must be above 70F, and it is difficult to tell if the conversion coat has been formed since it is a clear, although iridescent, coating. The color of the iridescence depends on the alloy of aluminum. While this may be Ok in a controlled production environment, this simply is too much for a shop environment. For these reasons, many users that have tried Aluminescent have switched back to Alodine.
Alodine comes in either a liquid or powder form. For an entire airplane, buy the powder. You can mix it as strong as you like. Alodine can be sprayed, brushed, or sponged onto large surfaces. The surface must remain wet for several minutes or until a light to dark golden brown develops. Alodine should not be allowed to dry on the surface because of the salts it contains. If this happens, the surface should be re-wet with Alodine. Smaller pieces that can be immersed into a plastic tub of Alodine often take less time. After the conversion process is complete, the part needs to be thoroughly rinsed with water. It is often recommend that the parts be primed within 3 days of the conversion coating. However, the reason for this is uncertain.
Information from Bondline.ORG