When attending the ZAC Workshop, we used a zinc chromate primer that
was water based, applying it with a brush and roller. Plastic
gloves were made available, as the zinc chromate is known to cause
cancer and birth defects. For a look at the hazards of zinc chromate
and zinc oxide, look at http://www.osha-slc.gov/SLTC/healthguidelines/zincchromate/recognition.html
When it came time to start my home building, I wanted to corrode proof the rest of the tail section, so I looked for some of the zinc chromate. ZAC didn't sell the water based stuff, so in the catalogs I saw the spray made by Tempo. This had the immediate advantage of being offered by two places close to me (Wicks and Beuco) but it has the disadvantage of using a toluene type vehicle which smells like airplane glue and is quite flammable.
Since then, I have heard or read the following about primers from other builders:
1. You don't need to use it - if Cessna doesn't do it, you don't need to either.
2. You don't need it unless you live near the sea.
3. You only need it where two pieces of metal join.
4. You need it everywhere, including dipping the rivets in it before you rivet.
The problem with 1 is that the corrosion happens - I spent some time looking inside some aircraft wings at a salvage yard, and it was there, both where pieces join and on open areas of aluminum, so if I follow #1, I would just be ignoring the problem. #2 assumes you know where you or anyone else who later might buy your plane is going to live in future, an assumption I don't want to make (but you might.) #3 ignores the corrosion that occurs on open stretches of aluminum skin for some reason - the video mentioned below shows several examples of that - perhaps since water helps to accelerate the corrosion and water has a longer drying time in between two parts that makes it worse, but I am not sure about that.
#4 Chris Heintz addressed in one of his talks. He said that if you apply a zinc primer on half a sheet of aluminum, the protection extends approx 1/4 inch past where the zinc stops. This is why small scratches are still protected, as well as the interior of rivets. Evidence of this can be seen from pictures I took of a plane made with blind rivets over 30 years ago. There is no rust from the center mandrel or any corrosion around any rivet I could find.
Much has been said of corrosion between aluminum in contact with another unlike metal, but then how does corrosion occur on a open piece of the aluminum inside a wing? Perhaps some metallic substance in rain water? I am not sure about this. I have heard A&P's attribute alclad 6061 aluminum with the same properties of gold, which I don't subscribe to.
Seems to me there ought to be a more definitive answer. For starters, I bought the EAA video entitled Corrosion and I have supplied a summary review of the tape here. My recommendation is that unless you are building a seaplane (there are lots of cool shots of seaplanes in it) you save yourself the money and read the review here and make up your own mind.
Another relevant question on all this is "How long do you want the plane to last? If you only plan for it to be flown for 20 years, and you keep it in a hangar that's not by the sea, then corrosion proofing might be a waste of time time and a few bucks. (In my case, about 8 cans times $7 is $56.00)
On the other hand, if you decide to sell it, will the corrosion proofing be a selling point? Since the most important place for it is between the riveted components, it is not something you can add later.
Bottom line on all this is that it seems to be a personal choice that you make based on how you feel about the arguments above. This is a sensitive topic, and it can start a flurry of traffic, both pro and con on many mailing lists. Just about everybody on those lists have endured it before, so be careful how you bring it up.
I decided that for my plane, I will add it since the time is not really that bad, and the cost is worth it for the peace of mind it brings. You may decide differently, and that's just as good.
Tempo also offers Zinc Oxide spray, which they describe as having the same corrosion proofing properties without the hazard, but it still has the vehicle smell and the airborne hazard of any spray.
The down sides of the corrosion proofing is the extra weight, the cost, and the biological exposure. I am going to use a mask, etc to minimize the exposure, which I will have to do to just paint the thing anyway. I am using the Tempo Zinc Chromate, which I weighed a full can at 15.5 oz. and an empty can at 4.5 oz. The label on the can says 11 oz, so I guess they are right! So, if there are 11 ounces that comes out of each can, I would need to know how much of the weight is propellant, how much is the smelly vehicle that evaporates, and how much is the actual zinc chromate paint that is sprayed.
Just to guess for now, I am going to say that half of the can is propellant/vehicle, leaving 5.5 oz of chromate sprayed. There is also a fair amount of overspray that does not make it onto the surfaces (make sure you use disposable drop cloth or something), I'll guess 10% of the total can. I will try to get better numbers for these guesses. This leaves 4.5 oz of zinc chromate per can applied to the surfaces. Now, I will keep track of the number of cans used, and that should roughly give me the weight of the corrosion proofing. Right now, with both wings and the tail and most of the fuselage sprayed, I estimate I will use 8 cans, which would be a total weight penalty of 36 oz. or just 2.25 pounds of applied primer for the whole plane. However, I believe this is a way too conservative calculation, so I will try to get better numbers.
One other note - in the photographs on this website, the primer appears much darker in the pictures than real life. I think this is due to the florescent lighting, or something biased in my camera. The technique I am using is to spray a solid coverage where two pieces meet, and then just apply roughly a 50% coverage over the other areas.
Images on this website are either Copyright Zenith Aircraft Company and used by permission or are copyright Gary Liming