Building, testing and operating an experimental aircraft

Surface Prep and Priming

03 Jan 2010

Well, today was the big day – priming day. I’ve been thinking about how I’d do this properly for nearly a year, so it was great to finally get started. I had read just about everything I could find on priming and corrosion control, and while I had initially considered going all out with alodine and a two-part epoxy primer, in the end I followed the logic of builders like Randy Lervold, who determined that a good self-etching primer was the best compromise between protection and ease of use.

SEM Self-Etching Primer SEM Self-Etching Primer

So I ordered two rattle cans of gray SEM Self-Etching Primer from Amazon, and decided to see how it worked out. Our prep and painting process is detailed below.

3-Step Surface Preparation

1. Dry abrasion with maroon Scotch-Brite pad to smooth surface anomalies and give the part some “tooth” for the primer. The part should have a dull, matte gray finish.

2. Abrasive wash and rinse using maroon Scotch-Brite pad, hot water, and Dawn dish liquid to remove surface contamination and aluminum particles left from step one. The hot water rinse should “sheet” off the surface, rather than beading up, indicating that it has been properly cleaned.

Cleaning with Dawn and Scotch-Brite Cleaning with Dawn and Scotch-Brite

3. Solvent degreasing after drying, using a wet cloth soaked in naphtha, followed immediately by a wipedown with a clean, dry cloth, to remove any residual oils and contaminates left from the previous step. Impermeable gloves must be worn from this point on, to keep skin oils away from the parts.

Degreasing with Naptha Degreasing with Naptha

Primer Application

It’s a good idea to read the material safety data sheet before using any product like SEM Self-Etching Primer. We picked up a pair of 3M 6000 Series respirators with 6001 organic vapor cartridges for lung protection and all-around Darth Vaderness. Eye and skin protection are recommended as well.

Pro Painters Pro Painters

Temperature, moisture and dust control are important for any painting process. As you can see, we had none of the above. The SEM has such a strong odor that we couldn’t bear to set up any kind of paint booth indoors, but temperatures outside today were in the single digits. Our solution was to spray each part individually outdoors – a process taking roughly 90 seconds per coat – then rush it back inside for drying before the primer started to freeze. Ideal? Hardly. But the SEM dries so uniformly that it seemed to work okay.

Drying Drying

The SEM dries well enough for recoating in five to ten minutes, and we put on three coats per part. We also prepped and primed the rivet lines on the inside of the skins, since we’d scratched up the Alclad so badly during layout and drilling. All in all, we used almost all of two 15.5 oz. rattle cans for the horizontal stab.

Finished Product Finished Product

I’m very happy with the end result, although we got a run or two and a few spots that could have used a bit more coverage. As you can see, even with the poor conditions, the SEM is very forgiving of amateur painting techniques. Next up, riveting!

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