I'm using 2 part epoxy primer from AKZO Nobel Aerospace - available from Spruce. I've chosen to do the entire alumiprep / alodine process.
In short, it goes as follows:
1. Clean all parts with solvent of your choice (I use MEK, as I am already "geared up" for toxic stuff later)
2. Dip or spray parts in alumiprep 33 (the gallon from spruce is mixed 3:1 to make 4 gallons) and scrub with scotchbrite pad. My understanding of this is that it is not to remove the alclad layer, but merely to scuff it and promote better adhesion of primer. I don't go overboard here. Rinse with water. The water should "sheet" off the part.
3. Before the part dries, it should be alodined. For small parts (stiffeners, small ribs, etc) I put a small amount of alodine in a plastic baggie and put the part in there for a short time. Larger parts get a wallpaper bin, and large spars, etc get sprayed with a spray bottle (note this gives pretty uneven coverage!). When the alodine is new, it takes 30-45 seconds to get the light golden color you're looking for. After having been used, it takes longer as the chromates are consumed by the conversion process. Note also that it must be kept from UV light or it will degrade as well.
4. Rinse again with water and allow the parts to dry. Ideallly the primer should be sprayed within 24 hours, so I let parts dry overnight sometimes.
5. AKZO is a 2 part epoxy primer, mixed 1:1. According to the MSDS you need to wait 30 minutes after mixing before you start using it. It is really easy to apply - you only need a light coat. Dries quickly in warmer temps, altho I sprayed a couple of days ago in 45F weather with no trouble at all.
I have tried various rattle cans, etc but the reason I am using AKZO is that it is truly bulletproof - I cannot remove it from parts when finished - not with solvent, scotchbrite, tools, etc. I am priming all stiffeners, ribs, etc. and I primed the inside of the skins of the control surfaces on the tail and wings - areas not easily inspected later. I really think that in the scheme of things this is minimal weight - I've used less than a gallon total, and a good amount of that was thrown away or oversprayed. I know that it is probably overkill in terms of priming, but I plan on keeping this airplane for a long time (I'm only 31) and know that I will never have to think about those parts in the future. I don't like the idea of spraying corrosionX or any other crap all over the inside of the plane - would rather put forth the effort now.
Thomas
-8 ailerons