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.
Here’s my process for preparing parts:
(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.)
- Remove factory plastic protective covering from the part
- Deburr sharp edges and holes (see the Tools page for recommended tools)
- Put a set of clean disposable nitrile gloves on both hands
- Spray or submerge part in a solution of 13:1 distilled water and Simple Green Extreme – this removes grease and other contaminants.
- 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).
- 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.
- 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.
- (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.
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.
- 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.