Internal Corrosion Protection

1. Peace of mind.
2. PPG recommends etching and applying a chromate conversion coating (Alodine or Iridite) for maximum primer adhesion and corrosion resistance. (See note about PPG's FaxBack service below for a copy of their tech bulletin on metal treatments.)
3. A MIL-P-23377F Type I Class 1 primer must pass a 1000 hour salt spray test with no adhesion loss, blisters or corrosion WITHOUT A TOPCOAT! (This requires proper surface preparation before priming including either Alodine, Iridite or anodizing.)
4. A local RV-6QB builder and EAA Flight Advisor gave me his extra copy of AC 43-4A "Corrosion Control for Aircraft". The glossy, color photographs of corrosion problems found on internal aircraft structures is enough to scare anyone into taking extra precautions to prevent corrosion.
5. Peace of mind.

I try to have all parts completely ready to assemble before beginning the preparation for priming. This means all holes match-drilled, holes and edges deburred, and holes dimpled or countersunk as required.

• If I am doing a small batch of parts or parts that are very clean, I skip this pre-cleaning step and go directly to the etching step shown below. However, for a large batch of parts (especially if they are contaminated with grease or oil), I preclean the parts with detergent. The PPG DX533 acid etchant contains a small amount of detergent, but when doing a large batch or greasy parts, you would have to change Scotchbrite pads and rinse water often to avoid contamination from grease build-up on the pads and in the rinse tanks. When pre-cleaning, I use Dawn dish detergent but I doubt if the exact product matters.

• Parts are then acid etched with PPG DX533 aluminum cleaner ($9.30/qt from auto paint store). DX533 is diluted with 3 parts water before use, so a quart goes a long way. The diluted cleaner is applied with a spray bottle while scrubbing the part with a fine Scotchbrite pad. Parts are then rinsed with water and allowed to air dry. You will notice that the rinse water will sheet over the entire surface if the parts are truly clean.

• The chromate conversion coating is next. Iridite and Alodine are two trade names to look for. Some auto paint stores sell premixed solutions in quart or gallon containers, but this can get very expensive if you need more than a couple of gallons. Pegasus Auto Racing Supplies (my wife and I are the owners) stocks a 10 pound container of powdered Iridite 14-2 which makes from 128 to 213 gallons depending upon the concentration desired. This is far more than you'll need, but you may be able to split it with other local builders. Smaller parts were soaked in a tray of Iridite for about a minute when the solution was fresh. The required soak time increases as the solution becomes depleted. Adding fresh solution will help to rejuvenate the solution and once again reduce the soak time. The darkness of the tint that is applied to the parts is a good indicator for the effectiveness of the coating. Larger parts were just swabbed with a rag soaked in Iridite solution. These didn't come out nearly as dark and uniform as the soaked pieces, but hopefully they are still somewhat protected. After the Iridite treatment, the parts were again rinsed with water and allowed to air dry.

  When working on the wings, I assembled the fuel tanks after the Iridite step was complete so that I wouldn't accidentally prime any of the internal tank parts.

  Applying Iridite solution to wing parts:

  
  Using 4 x 4's screwed to the workbench and lined with several layers of heavy plastic, I made a tray that was just big enough to fit the wing ribs into. I applied the conversion coating to almost all of the small wing parts at one time using this tray. Each part was agitated in the solution for about one minute. The blue bin contains rinse water. It gets slightly tinted from the residue of Iridite solution on the parts, however, I am quite sure that this is too dilute to continue the process. If you soak the parts too long in the rinse water, the tint will fade and the coating will become less effective. The parts were later rinsed a second time and then allowed to air dry prior to priming. The used Iridite solution was siphoned back into a sealed container. I have reused it several times after adding some fresh solution to make up for what was lost. It still seems to be effective after several uses, however the required soak time increases as the solution becomes depleted.

• I am using an epoxy primer that meets MIL-P-23377 Rev. F Type I Class 1. I obtained this from PRC-DeSoto (a division of PPG). I ordered it from their Indianapolis sales office: (317)290-1600. The primer base is part number 513X390 and the activator is part number 910X624. These both come in gallon containers ($89.78 for a 2 gallon kit). They also suggest adding a special (very expensive) thinner before spraying which is part number 020X411 ($69.95/gal). The mix ratio is 4:4:1 by volume (base/activator/thinner). Their minimum order is $100. I heard a rumor that this epoxy primer may be discontinued because of EPA rules on VOC's so I ordered a second kit to make sure that I have enough to finish the airframe. It only has a 1 year shelf life so I have to work fast! (I have the extra kit in the refrigerator to help extend the shelf life - much to my wife's dismay!) It looks like a single 2 gallon kit would fall slightly short of completing the airframe interior. I am applying the primer with a Sharpe Cobalt HVLP spray gun. When priming the interior surfaces of the skins, I put a heavier primer coat along the rivet lines where the skin would be in contact with the underlying structure. However, in order to save weight, the remainder of the internal skin surface receives only a very light coat. Only a thin, semi-transparent coat is required to meet the Mil Spec for corrosion resistance (optimum film thickness is 0.6 to 0.9 mil). One more reminder: you need to let the primer sit for 30 minutes after mixing the components before beginning to spray.

  The spray equipment cleans up easily with lacquer thinner. I also found that any overspray on the exterior of my skins was easily cleaned off with lacquer thinner during the first 24 hours or so. If the overspray is allowed to cure for several days it becomes very difficult to remove. According to the spec sheet, 7 days is required for a full cure.
  

• PPG has a FaxBack service with information on many of their products. Call 1-800-450-2654 and follow the prompts. One tech bulletin that I found helpful was document #7226. This covers metal treatments including etching and chromate conversion coatings of aluminum for maximum corrosion resistance and adhesion of primer.