How to Prep & Prime Parts

I use the AKZO Fast-Drying 2-part Epoxy Primer, which is available from Aircraft Spruce for about $100.

It comes in a 2-gallon 2-component kit...one gallon is the green stuff, the other is the catalyst. You might think $100 is expensive, but it works out to be considerably cheaper than the rattle can based primers.

What do I use to spray it on? A $19 touch-up spray gun from Harbor Freight Tools.

Here's a question I got in an email message:

> Question:  what process do you follow that has worked for you to prepare the
> Aluminum before you shoot the AKZO primer?  MEK, Alodine?  Do you also shoot
> the skins since they are alclad?

I'll answer these questions in reverse order...nope, I don't shoot the insides of skins. You might think I'm nuts, but I believe the Alclad, if kept unscratched (worth all the trouble with the soldering iron to strategically leave on patches of blue plastic while working with the skins), will last a good long time before any corrosion forms. I prime anything which has been scratched, filed, extruded, flanged, or pressed (notice how Alclad thins in bent areas, like on rib flange corners?).

In answer to "MEK, Alodine?" I don't believe Alodine will be effective on Alclad, even if you clean it with MEK. I'm pretty sure Alodine only works effectively on raw aluminum. So I "etch" the parts first.

Here's what I do as the full priming process...this process is what was recommended to me and I'm convinced it's the best way to go. I apologize if anything in here is way too detailed (like "transfer the primer into your gun"), but you never know. Some people like lots of detail...

1. Set up hangers for all the pieces you'll be prepping. Not necessarily hangers where the parts will get sprayed, but where they can dry during the prep process. I usually use a bunch of pieces of stripped strands of 12 gauge (or whatever) Romex, which holds its shape nicely when bent and fits through #30 holes no problem:
   

   Sometimes I just hammer some nails into a 2x4 clamped across whatever:
   

   Other times I just use nails in the edge of a bench:
   

2. Set up the prep area...I have a drain in the driveway just outside my garage. Don't even tell me how horrible it is to let chemicals wash into ocean...I feel bad enough about it already...I just try to minimize the amount of chemicals I drain. I've pretty much accepted that during the building process I'm contributing to the demise of the environment. Ok, guilt trip aside...anyway, I bring a hose over for rinsing. Get a little stool or something to sit on to make your life a little easier. Get a maroon Scotch Brite pad (7447 or whatever). Wear full cover, a respirator, saftey goggles, and rubber gloves. I want to take no chances with Alodine, a chromic sure to cause nasty health problems.

3. Dilute Alumiprep in a spray bottle according to the instructions. Take each part, spray it down with the Alumiprep solution, and scrub the bejezus out of it with the Scotch Brite pad. There's nothing like the look of RAW aluminum. It has that dull smooth character once the Alclad has been stripped off. Get the edges, too. Don't let the Alumiprep dry, do one side or portion at a time if you have to. Rinse the part off thoroughly when you're done and hang it back up.

4. Put a small bit of Alodine in a cup and get a foam brush. Somehow brush the Alodine onto the parts. My technique varies here...I used to brush it on while the part was hanging but that's kind of silly since the stuff runs right down and drips off, and having to raise your arm to brush it on you're likely to get Alodine running down the glove and on your sleeve, increasing the chance of contacting that nasty crap. Lately I just lay the parts down horizontally on a cardboard box or a chicken wire mesh and brush the Alodine on there. I believe the best way to Alodine stuff is in a dip trough, though (although I never do it). That ensures the best consistent coverage and gets you that nice gold finish...but it takes FAR more Alodine to do that. I've seen people design all sorts of recycling methods for Alodine, and that's a good way to go if you want to put the extra effort in. I've also seen people fill a PVC pipe with Alodine in which you can soak long, narrow parts like stiffeners without using as much. But for larger parts like spars and ribs, I'm not about to fill a huge bucket full of Alodine if you know what I mean. I'm satisfied with the level of conversion that brushing on and using very little Alodine provides.

   My latest trick for small parts is to put about 1/4 cup of Alodine in a large Ziploc bag and bathe the parts in there...it wastes basically no Alodine, provides excellent coverage, and simplifies handling and recycling the Alodine. To each his own here. Anyway, let the part sit/soak/brush for 3 minutes or whatever the instructions say, and then thoroughly rinse the part off and hang it up to dry FULLY. It's at this phase where nothing should touch the part, especially not oily fingers. Use latex/rubber gloves when handling the parts between now and spraying.

5. Once everything is dry, get it into position for priming. I don't have a consistent way of doing this. Sometimes I lay stuff across chicken wire, sometimes I spray it while hanging, sometimes I lay it out on a dust-free surface. Whatever works for you.

6. Mix up the primer. My understanding (and this could be completely incorrect) is that there's a 30-minute "induction" period for AKZO...and my interpretation of that is that you mix one-to-one of the green stuff and the catalyst, and then you let it sit 30 minutes while it does its chemical thing. I mix it every 5-10 minutes or so during that process with one of those drill-mounted stirrers. I'm guessing that the 30-minute thing is either wrong or unnecessary, but who knows. I'm not a chemist, but it has worked like a charm for me and I'm happy with how my primer layer comes out.

7. Get the spray gun set up. The 30-minute "induction" time is a good time to get last-minute items ready. I take my inline oiler out of the air line and hook up a dedicated "clean air" line for spraying. I set my regulator on about 23 psi (the magic # for me, you'll find your own). Get some latex gloves handy. I even put ear plugs in when priming so that chromic crap doesn't get in my ears and have a nice straight path to my brain. Anyway, once the primer's ready, get your safety gear on and transfer the primer to your spray gun -- I use those cheap little disposable paper funnels from Spruce. Have some Acetone and a rag around for cleanup.

8. Spray everything. A thin, semi-transparent layer is all that's required. I tend to go a little heavier than that just for warm fuzzies. I like the way the AKZO holds up to scratches, chemicals, etc. You'll find your own happy medium thickness-wise.
   
   Are you sitting there thinking I'm nuts for priming parts so close to my black Honda? Not an unreasonable assumption, but there's virtually no overspray whatsoever with this combination of primer and spray gun. I hold the gun about 8-12" away from the target and it still doesn't overspray noticeably. Yeah, I'm probably nuts, though. 8^)

9. Let it dry at least 15-30 minutes before handling. If you can let it dry in the sun, it will be ready for handling almost instantaneously. AKZO dries very quickly in dry heat, especially direct sunlight.

10. Cleanup: I clean my spray gun with Acetone. First I fill the cup up about halfway and then spray it on some scrap (i.e. a cardboard box, into a trash can, whatever) while shaking it vigorously, cycling the "mixture", etc. Once that's all sprayed out, I take the cup off and wipe everything down as best I can with Acetone, paying close attention to the nozzle, etc.

Yeah, this is absolutely more time-consuming than just MEK'ing the parts down and spraying on some self-etching primer from a can. But for me, AKZO is as durable as I want it to be. I don't dig the way the spray can primer lays on. Much less durable in my opinion. Take all this with a grain of salt.

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Dan Checkoway ()