Suffice to say, that there are a lot of opinions about what needs to be primed on the inside of an RV airplane, and whether we need to prime at all. It is pretty clear that there is no way to squelch this controversy once-and-for-all, so I will just tell you what I am doing. For another site that does a pretty good job of looking at this, see Primer Wars on Andy Karmy's website.
I agree with Van that 2024-T3 aluminum, with its Alclad coating, is sufficient protection against corrosion, and we don't need to prime -T3 parts unless we compromise the Alclad coating. Having said that, it is virtually impossible to work with these parts to any degree without scratching the the Alclad coating. I leave the plastic on the skins as long as possible, so they seem to be pretty well protected, even after drilling, dimpling and deburring. Therefore, I am not priming the inside of the skins unless I scratch them. The ribs and other flanged parts do not fare as well, so the ribs and spars, and just about everything else will receive a primer coat.
Originally, I didn't want to mess with mixing epoxy and doing clean-up after priming parts. I would have to accumulate a lot of prepared pieces to make it worth my time before priming, and I like to put things together as soon as possible. Therefore, I decided to go with the etching primers in spray cans. There are several choices but the two that I considered were Napa 7220 and Sherwin Williams 988. Since the local Napa parts stores didn't know what 7220 was, I went with SW 988, plus an enamel topcoat. I used 988 on a number of pieces, and it seemed to work ok, but I'm pretty sure that the enamel topcoat wasn't as hard as epoxy.
I also experimented with MetalPrep79 acid etch and Alodine. These can be ordered from Aircraft Spruce and Speciality. The Etch/Alodine process does a good job, but it is a lot of work, so I started to lean towards the epoxy solution, again. Then, I discovered the one-part primer/sealer from Aircraft Finishing Systems . This can be applied without a conversion coating (Alodine), but it does require an acid etch (i.e., MetalPrep79 or their own Acid Etch). The really nice thing about this primer is that it is thinned with water, and clean-up is with water. You can even pour the unused paint back into the bottle after spraying. This makes it a lot more convenient to use with just a few parts.
I have now painted quite a few parts with the AFS primer, including all of the parts on the wing except for the skins, and I am really happy it. The clean-up and the ability to pour unused primer back into the bottle makes it really easy to use and almost waste free. It is nearly as convenient as the rattle-can primers. Since it is a primer-sealer, it doesn't have to be top-coated in order to provide protection, so it is similar to the epoxy primers in this regard. Note, that a lot of builders are just spraying 7220, 988, or Mar-hyde on their parts, thinking that they are protecting them. I really wish they would read the literature and realize how useless this is. These primers offer virtually no protection without a topcoat.
The AFS primer is a lot more durable than the enamel top-coat over the 988 that I used on the empennage. When I did the ribs for the wing, I primed them all at the same time. However, I built the wings separately, so the second set of ribs got quite a bit of handling before they were installed. The primer really held-up well on the second set of ribs.
The only downside that I see with the AFS primer is that it takes a little longer to dry, and it is not impervious to MEK. Even after it had dried for several months, I was able to remove the primer with MEK without much rubbing. Lacquer thinner will clean-up the primer right after it has been applied (this is good). After a few months you can wipe down the parts with lacquer thinner, and the primer doesn't wipe right off, but it would probably come off with a little rubbing. I need to see what gasoline will do to the primer after it has fully cured.
When I went looking for an external paint solution, I had a lot of trouble with the major players, such as PPG Industries, and Sherwin Williams. I will not bore you with a long explanation of why I did not chose them. I will simply say that either I could not find the information that I needed, or I could not find a supplier that would deal with an individual. SW did send me a nice brochure with a color chart, but it was a little late, since I had already chosen Aircraft Finishing Systems (AFS).
The best thing about AFS is their website. This is not a flashy, "big production" website, but it does have several things that the big guys lack. First, it has the best explaination of what their paint system consists of, and how it is applied. Second, they have a price list of all of their products. And, finally, they have a flipping color chart on their website. I know some people will say that a computer will not be able to show an exact representation of actual colors--you need a hard copy. I say, with their color chart, at least you can get an idea of what colors are available, and you can get a real color name that can be ordered.
AFS makes a Waterborne 2-Part Polyurethane Topcoat, and a Waterborne 1-Part Primer/Sealer. Supposedly, these are both environmentally friendly, and they both clean-up with water. The Primer/Sealer can be applied direct to metal, as long as you have properly cleaned it and done an acid etch. And, very importantly, the Primer/Sealer is a sealer, so it does not have to be topcoated when you use it on the inside of an airplane.
I placed an order with Stewarts Hanger 21, in Cashmere, Wa., who is one of AFS's distributors. I ordered a gallon of their 1-Part Primer/Sealer, and 3 quarts of topcoat (Insignia White, Metalic Red, and Metalic Blue). I received half of the order from Stewart's Hangar in 4-5 days, and the other half arrived direct from the factory in about a week and a half. The paint was all well-packaged, and arrived without any damage. I even got a copy of their local newspaper, the Missoulian. A nice thing about the paint is that both the Topcoat and the Primer/Sealer come in plastic bottles. This makes it a lot easier to work with than a typical tin can.
I really don't have my shop setup with a proper paint booth, but I couldn't stand looking at that paint, without wanting to see just what it would look like on my airplane. So, I finished sanding the fiberglass tip for the right elevator, and proceeded to attempt to paint it with the Metalic Red topcoat. I have a $25 Harbor Freight siphon paint gun that I got on sale a few weeks ago for $16. The gun has a 1.8 mm tip, and AFS recommends a 1.0-1.2mm tip for their topcoat--whatever. Also, my only experience with this paint gun is the 5 or 6 times that I have practiced painting the inside of a cardboard box over the last few weeks, using interier and exterier latex paint--not exactly a paint pro.
Well, as you can imagine, my first attempt at painting the fiberglass tip was less than spectacular. After the fog coat, I waited 5 minutes and then I applied a little heavier coat, per the instructions. I waited 5 more minutes before applying the wet coat. That's when I noticed a lot of little circles in the paint surface. I thought this might be fisheye, which usually indicates that there is dirt or oil on the surface or in the paint. I was pretty sure that I had not cleaned the fiberglass properly, but I went ahead and applied the wet coat, anyway. I poured what was left of the 1/4 cup of paint into a plastic cup and I cleaned the gun. I considered putting the left-over paint into the freezer, but I didn't--hey, this paint is $85 a quart ($286 a gallon).
I checked the paint about 30 minutes later, and there was still a lot of what appeared to be holes in the paint. I thought I would try to spray another coat, so I poured the mixed paint back into the spray gun. I should have known this was not a good idea, since it was obvious that the paint was starting to setup, but hey, I wanted to "save" my paint job. The paint did not spray as good as the first few coats, and when I went to clean the gun again, I thought I had blown it. This paint does not clean-up with water after an hour. The good news is that MEK still does the job if the paint is still wet, so my $16 spray gun is still ok.
Later that night, after the paint had dried for 6-8 hours, I took another look. The surface was nice and shiny, and the color was a pretty dark red, just what I wanted. A lot of the little holes had closed up, and there were no holes that went all the way to the fiberglass. There was a lot of orange peel, and the surface was definitely not acceptable. I decided I would repaint the part the next day, so I sanded on it for awhile until most of the surface was dull. Note, that there were still a few places where the surface coat did not get sanded away, and I figured that I had screwed-up again, but it was time to go to bed.
The next morning I built a pedestal to hold the part while I painted it, rather than suspending it from a hanger and letting it flop around. This time, the results were almost perfect. The first two coats did not show any signs of fisheye or orange peel. After the wet coat, I noticed two little circles, but that was all. Also, since I put the paint on a little heavy, I figured it would run a bit, but I hoped that it would just collect at the bottom edge, which would be hidden inside of the elevator. I was very happy with the results, but I still had to wait for things to dry before I could declare success.
The final result was that the two circles almost closed up, and the runs were confined to the bottom edge of the part. I did have to sand the runs a little, and I will have to spray again, but as a first attempt, with a $16 HF spray gun, I am very happy. The paint itself looks absolutely beautiful, and the coverage is very good. With a little practice, I think I will be able to spray my own airplane. It's clear that this will not be a show plane, but then, I don't have the facilities and equipment to produce a perfect job. However, if the rest of the airplane looks as nice as this first part, even with its inperfections, then I will be quite satisfied. So far, I am extremly happy with this AFS paint.
For those of you who are still with me, here are a few paint schemes that I am considering. Note, that this is an RV-6A, since the paint program that I am using from Aircraft Paint Schemes did not have an RV-7A, but it's close enough.
Stay tuned, more to come...