November 18, 2019

Process

Preparing & Priming Parts

I chose to prime the aluminum parts to protect against corrosion.  The Sling 4 aircraft is constructed from 6061-T6 aluminum, a corrosion-resistant grade of Al.  However, the relatively high moisture levels of New England and proximity to sea air (I’m only 20 minute flight to the ocean) made this extra priming step necessary, in my opinion, to ensure long life and prevent costly repairs later.

The primary type of corrosion I’m trying to prevent is galvanic or electrolytic corrosion, which can occur when the metal is exposed to moisture, and electrical currents flow though or across faying surfaces (i.e, where two pieces of metal contact, such as rib-to-skin contact).  The primer acts as a barrier to the moisture and electron flow.  Therefore, I’m primarily priming parts at the mating/faying surfaces.

There’s a bunch of articles on corrosion here, courtesy of EAA Chapter 1000.

Some general tips for priming (based on my experience):

  • The metal surface has to be absolutely clean – make sure there aren’t any residues left behind from your chosen cleaner. Pay attention to bend corners on flanges.
  • Primer will stick better when the surface has been lightly abraded with 320-grit sanding paper or maroon (ultra fine) Scotch Brite pads
  • Don’t try to prime the firewall – this part is galvanized, which most Alkyd-based products have trouble adhering to. The galvanization should offer enough protection.
  • There are some parts of the Sling kits that are already treated by the factory (e.g. main wing spars), priming these parts is not necessary
  • Moisture is the absolute worst enemy to good primer adhesion! Be sure to follow the product directions on your primer.
    • You have to work within the temperature and humidity range of the primer you’re using; generally 50-80°F and <65% relative humidity. I’ve been able to work up to 75% RH, but you need to use really light coats, and allow minimum 10 minutes drying time between coats.
    • Don’t try to prime parts during periods of rapid temperature change (morning/evening), unless you’re working in a temperature- and humidity-controlled environment; the air temperature/humidity can change more rapidly than a thermometer/hygrometer can register during these periods.
    • Don’t prime if it’s raining or about to rain – the humidity level can change rapidly
  • Bottom Line: If it’s too hot, too cold, too humid, or raining, don’t try to prime. You’ll just waste time later having to remove the flaking primer with sandpaper or acetone and start over!

NEW PROCESS (as of 8 October 2019):

(This is a new process based on the recommendation of SEM that makes the primer I’ve been using.  Despite many refinements to my old process – which I kept below – I still occasionally had problems with the primer flaking off. I found that using SEM Solve is much faster and more reliable than the other cleaning products/methods identified in the old process)

  1. Remove factory plastic protective covering from the part
  2. Deburr all sharp edges and holes  (see the Tools page for recommended tools)
  3. Use a maroon Scotch Brite pad to lightly abrade the surfaces to be primed.
  4. Put a set of clean disposable nitrile gloves on both hands.
  5. Spray SEM Solve directly onto the part where it will be primed. Be sure to use proper protective gear (gloves, respirator mask), following all precautions on the product label.
  6. Dry the part with a clean towel. I use yellow Costco micro fiber cloths that come in a large multi-pack. SEM Solve is a volatile solvent-based cleaner, so you should see it “flash” off the surface of the part as you wipe it down with the towel.
  7. If there is any residue on the surface from glue, labels, or tape, repeat steps 5 & 6 until clean.
  8. (Optional) In certain conditions (high relative humidity, or low temperatures), you may want to further dry the part with a heat gun to ensure all moisture is gone.
  9. (If desired) write the part number onto the part to help identify it later. Use a Sharpie marker.  The ink should easily stick to the metal if it has been properly cleaned and dried.
  10. Prime the part using your chosen product. I use SEM Self-Etching Primer that’s sold in 15 oz rattle cans. Use multiple, light coats, waiting 5-10 minutes between coats (depending on temperature, humidity). DO NOT try to put on a thick coat in a single pass, it WILL eventually flake off in a day or two.
  11. Allow part to dry for minimum of 30 minutes before handling. I recommend at least one day of dry time to allow the primer to cure.

Note: SEM Solve won’t dissolve the black identifying information that’s sprayed onto the metal; if you wish to remove that, apply some acetone to a rag and wipe it off before step 5

OLD PROCESS:

(Note: on 20 April 2019, I inserted step 7 (acetone clean), and removed step 9 (isopropyl clean).  I found that isopropyl doesn’t clean as well as acetone.) 

  1. Remove factory plastic protective covering from the part
  2. Deburr sharp edges and holes  (see the Tools page for recommended tools)
  3. Put a set of clean disposable nitrile gloves on both hands
  4. Spray or submerge part in a solution of 13:1 distilled water and Simple Green Extreme – this removes grease and other contaminants.
  5. While part is still wet with the cleaner, use a Scotch Brite pad to lightly abrade the surfaces to be primed. I used the green (medium grit) pads.  You could also use a fine-grit sandpaper (400-600 grit).
  6. Rinse off your hands and the part with clean water, and then dry the part with a clean towel. I use yellow Costco micro fiber cloths that come in a large multi-pack.
  7. Apply acetone to a clean, folded towel and thoroughly clean the part.  Be sure to use proper protective gear (gloves, respirator mask and/or fresh air).  Use a second clean towel to wipe off any residue that the acetone dissolves (e.g. ink).  Re-fold the first towel and wipe again with acetone if necessary to remove any remaining residue, then wipe clean with the second towel.
  8. (If desired) write the part number onto the part to help identify it later. Use a Sharpie marker.  The ink should easily stick to the metal if it has been properly cleaned and dried.
  9. Spray isopropyl alcohol (at least 70%), onto a clean, folded microfiber towel and wipe down the part.  On larger parts, turn the towel and re-spray it as you move from section to section.  Be sure you’re still wearing gloves for this step and that they’re clean, otherwise you risk contaminating the part.
    If there is a lot of residue on the towel after wiping, repeat this step with a second clean towel until it comes clean.  The surface of the part should feel squeaky clean if you run your gloved finger over the surface.
  10. Part is now ready to be primed – follow the directions of your chosen primer product.  I use SEM Self Etching Primer in a rattle can.