Boy, if there is one topic in building an RV sure to start a discussion, it is the topic of priming.  After some significant thought, I have decided to prime majority of my aircrafts internal surfaces.  Not to mention, I have chosen a method that is one of the most time consuming of all popular methods utilized by RV builders.  I am using AKZO's two part epoxy primer.  This stuff is as "tough as nails".  However, priming has added significant construction time to the project.  In one case, I lost 4 hrs to re-fabricate 1 stiffener and get it primed.  If I wasn't priming, I would have only had a setback of around 0.5 hr.  So if you decide to prime your project, expect to spend a lot of time just priming (at least if you chose the two part epoxy primer method).

I have been VERY PLEASED with my results (other than my dysfunction with learning to use the spray gun).

First of all, I clean the aluminum with Alumiprep 33.  This is mixed with water to make up the solution.  Henkel's instructions state to mix 2-3 parts water for 1 part Alumiprep.  I let the aluminum parts set in the "tub" for 1 to 3 minutes and you can see the aluminum "fizzing".

 
http://www.aircraftspruce.com/catalog/cspages/alumprepaint.php
 
Once you have let the chemical action begin, I scrub the parts with a scotchbrite pad.  I tend to use the more aggressive scotch-brite pads you see on the left.  I got these from Cleaveland Tool.   I scrub the aluminum until it has a uniform brite, but dull appearance.

Rinse heavily with water until all soapy/bubbly "alumiprep" solution has been rinsed off.  The Alumiprep bottle says to rinse until it sheets off the material.

I dry with a relatively lint-free towel and let set to fully air dry.  Once I have started the Alumiprep process, I no longer touch the aluminum with bare hands (mainly for the contamination by the oils on your hand).

Once the part is dry, I begin the alodining process.  I use Alodine 1201 which gives the aluminum a gold/brown appearance.  (I had another RV builder criticize me once because I used the term "brown" instead of "golden".  I guess I am "lazy writer" as I seem to use those two colors interchangeably.)

http://www.aircraftspruce.com/catalog/cspages/alumprepaint.php

The trick to alodining is that it is BEST if the part can be SUBMERGED in an Alodine bath.  The instructions on the bottle say that it can be brushed on, however, I have been VERY DISAPPOINTED with the results of that method.  When you rinse the part, you can barely tell you alodined at all by brushing.

When I was looking for tubs that I could use to submerge the part in without having to get a 55 gal drum of Alodine. 

 
One unusual thing that I found to work great to rinse parts off without splashing alot (when I can't work outside due to the cold weather) was a TURKEY BASTER.  (My wife's idea)

I found that a Sterilelite(?) (rubbermaid) for christmas gift wrapping paper wouldn't hold a lot of Alodine and would also allow submerging larger ribs in the wing kit (about $10).  Some builders have made there own custom tubs with 2x4's and vapor barrier liners.

The tub below, was originally sold at Menards as a cat scratching device.  It had a corrugated cardboard box treated with catnip.  I pulled the cardboard out and gave it to the cat and kept the box for Alodining and Alumi-prepping.  I have two of these tubs.

I am doing all of this alodining with 4 gallons (I plan on priming majority of components throughout the construction process- Just starting my wings).  If I knew then what I know now, I could probably start/get by with 3 gallons, but would have to move Alodine back and forth to the different tubs and PVC tube more often.  I only bought 1 gallon of Alumiprep 33 and I should have bought 2.

Once the part is done in the Alodine bath, rinse with clean water and I let it drip dry as you can wipe off a lot of the golden/brown color until the part dries.  The trick here is not to let it set to long in the bath.  I pull it out when it has a nice light golden color as it darkens when it dries.

Again, I don't let bare skin touch the part as I am priming with a two part AKZO Nobel Fast Drying Interior Epoxy Primer.  I found that a spool of suspended ceiling wire is great to bend and form hooks to set parts into and out of an Alodine bath and then hang them up to dry in my paint booth.

http://www.aircraftspruce.com/catalog/cspages/primers.php

Excerpt from Aircraft Spruce Web Site*******************

AKZO interior epoxy primer #463.12-8 is a chemically cured 2-component epoxy primer designed to provide maximum protection from various chemicals, hydraulic fluids, aviation fuels, skyrol fluids, and corrosive causing media.  Drying time to touch is 15 minutes and to re-coat is 1-24 hours.  Shelf life: 2yrs, Color: Green.  used extensively in military and aerospace applications and popular with aircraft builders.  Uses 1 gallon of primer & 1 gallon of catalyst.

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The epoxy primer is mixed one component 'A' to one component 'B'.  There is an "induction time" of 30 minutes which I figure is the amount of time I need to let the primer sit to do its chemical thing.   After the 30 minutes have elapsed, I pour the primer into the spray gun and go to work.

When mixing, I use a set of Pyrex mixing cups to measure out the epoxy primer.  They work really well and clean up easily enough (as long as you don't let the epoxy primer cure while on the pyrex).  I got them at Target for around $25.  This is the set that I purchased.  I gave the egg holder and the measuring spoons to my wife and I use the rest.

For spraying, I use the DeVilbiss HVLP (high volume low pressure) gravity feed spray gun (model FLG-635).  I wanted a HVLP spray gun as they are claimed to be a lot more efficient at placing the paint/primer on the material with minimal over spray.  At a $100 a kit for primer, I don't want to waste a lot of it priming the garage.  I owned a suction type spray gun and I was never that impressed with its ability to draw up paint out of the cup.  I am spending $$$$ on the plane and I don't plan on skimping on tools to build it (within reason, of course).

I bought this spray gun at Harbor Freight for $140.  I have been really happy with its performance.  It wasn't the cheapest HVLP gun out there, but I can get support and replacement parts for it.  Good thing!!!  I already about toasted my fluid needle (my fault, see my left elevator log entries).

Replacement parts can be found at:

http://www.spraygundepot.com/devilbiss_files/finishline.html

At the time I inputed this into the web page, spraygundepot.com offered this sprayer for $129 which is cheaper than Harbor Freight.

If I could change one thing about this spray gun, it would be to buy the next model up with the metal handle.  This handle on this gun is solid, but it is a little hard to remove the epoxy primer from.  Otherwise, no complaints...

Be prepared if you can't buy aluminum prep and priming chemicals locally to pay some pretty hefty shipping charges.  When I upgraded my plans on Alodining, I ordered 3 gallons of Alodine from Aircraft Spruce.  The Alodine cost me around $45 and the shipping was $60 and that was by the cheapest shipping method available.  Same holds true for any Alumiprep you ship.

For the empennage, I used right at 1/3 of a gallon Alumiprep (I usually mixed 3:1 and got great results) and 1/4 gallon Alodine.

PERSONNEL PROTECTION:  I wear long sleeve clothes when working with these chemicals, and wear latex gloves under a set of Nitrile gloves or vinyl gloves.  Sometimes, I double-up my latex gloves.  With any combination, I need to change gloves about every 15-20 minutes.  These chemicals seem to eat these materials and you can tell you need to change when the gloves started sticking (almost like "melting") when they touch together.  If my clothes get splashed, I remove them and change immediately.  It would not be a bad idea for goggles or at least safety glasses either.  I try to be very careful, especially when scrubbing with the Alumiprep as it is real easy to splash alumiprep solution all over.  After all is said and done, I clean up and ventilate the garage (I usually spray primer in the same session).

Below is a set of latex gloves that Dan Checkoway raves about on his web site.  I have found that there is a big difference in latex gloves.  I bought a cheap box at Target (they were junk), I bought some more expensive ones at Home Depot and I like them except they are powdered.  After prepping your aluminum, you are now introducing foreign contaminants to the objects being primed.  These gloves are GREAT.  They are a really heavy weight latex glove and they are not powdered.  I go through significantly less gloves when undertaking aluminum scrubbing activities than I did with the cheap latex gloves.  I also keep around cheaper latex gloves for my clean up activities so that I don't go broke using these.  They are relatively expensive as far as latex gloves go.

They can be bought over the internet at the following link:

http://www.allmed.net/catalog/showitem.php/3694

I started with just an organic vapor respirator.  It works pretty well.  You can get these at Menards, Home Depot, Lowes, etc.  They are about $35 and each set of organic vapor cartridges is around $18.  I went through 2 sets of organic vapor cartridges during my empennage kit priming efforts.  This type of respirator is nice as it allows you to roam the garage without being tethered to any hoses. 

After careful consideration, I decided that I wanted a try what sounded like a better solution.  First of all, the 3M respirator was just a 1/2 face respirator.  Even with goggles, I was concerned about primer getting into my eyes and after at least one priming session, I was hacking for about the next day.  With the Hobby Air system below, the box sits outside in clean air and a turbine (fan) provides low pressure air to the face mask.  By using a full face mask, the problem with getting primer in my eyes is gone.  The Hobby Air system keeps the face mask at a positive pressure.  This way if there are any leaks around the face seal, hopefully it will be air escaping and not vapors coming in.  It's downside, however, is that you are now tethered to a hose.  The hobby air system was around $465.

The Hobby Air system works pretty well, however, the air hose (white hose) is very heavy and stiff to haul around.  If you are in a small space, you will probably spend half of your time tripping over this hose.  Maybe a compromise in a respirator would be to use a full face organic cartridge respirator.  This way you get some better eye/face protection and the freedom from the heavy air hose.

FINAL REVIEW:  Works, but big initial outlay of money and the heavy hose leaves me reaching back for my 3M cartridge respirator.

The degree you chose to prime/protect is a personal choice.  I know Van's requires some parts to be primed in the -7 kit.  I have chose to go above and beyond the minimum.  I guesstimate that I have between 1/4 to 1/3 of my project time on Alodining and priming activities alone.