Primer

Most people think that aluminium, from which the RV-9 is built, does not corrode. Unfortunately, this is not quite the case. It is true that pure aluminium reacts with air to form a protective coating of aluminium oxide which is highly resistant to corrosion. In this way, pure aluminium does indeed protect itself. However, the alloys of aluminium used in building aircraft do not have this feature. Some of the parts of the Vans range are coated at the mill with a thin layer of pure alluminium to restore this property. These are refered to as "alclad". The problem is that the coating is very thin and prone to scratching, leaving an entry point for corrosion. You can easily recognise alclad parts because they are covered in a light adhesive film of blue plastic to protect them from such scratches. At some stage however, this film has to come off. Then there is the large number of parts (including all pressed parts such as ribs and bulkheads and the lengths of stock sections used for stringers and longerons) that are not alclad in the first place.

Many US builders debate whether to prime their planes at all. Their warm dry climate may give them a choice but our atlantic climate gives us none at all. I don't know of any builder on this side of the pond who is not priming. I don't have hangerage for the plane sorted out yet and there is a distinct possiblity that it will be stored in the open for at least a period after it is finished. Consequently, it would not be sensible to leave corrosion protection to chance. That disposes of the first battleground of the 'primer wars', which is whether to prime at all or not.

The next question is where to prime. Well the counter question is why wouldn't you prime everything? The answer to that is three-fold; time, cost and weight. These are the main arguments quoted by those who do not favour priming. For me, it is a question of balancing these factors against the need for additional corrosion protection.

One option is to prime everything that is not alclad. Even on an alclad part, the cut edges are not protected. In addition, you are drilling holes through the metal for rivets and sometimes cutting a part such as a stiffner to length, thereby creating more edges. So you would have to prime these areas anyway. Then there is the possibility that condensation will form inside the fuselage and will gather in crevices in the structure. This is where corrosion usually starts. These crevices will normally be at points where two metal parts are joined so it makes sense to prime at all such points. You could, for instance, prime along the narrow strips onto which stiffeners will be riveted. This would ensure that the rivet holes in the skin would be covered also. Even alclad parts riveted to the skin (such as stiffners) have edges and rivet holes where the aluminium alloy is exposed. As they are so small, it would not be worthwhile to prime only holes and edges so in practice the entire part must be done.

At this stage, you have to ask yourself, what areas of the aircraft structure would not be primed under such a regine and the answer is that it would amount to the central areas of the skin that are not close to an edge or close to another part. I started out this blog with the position that I would adopt this strategy but now I feel that leaviing these areas would be more trouble than it was worth.

The rivets themselves have always struck me as being a likely place for corrosion to get a foothold and also the one place you would not want this to happen. The only way of protecting these areas is to 'wet rivet', meaning that every rivet is dunked in primer before being set while still wet. I am interested in giving this a try. If I can develop a workable method, I will use it. Otherwise a compromise may be on the cards.

The next issue is which primer or primer system. There is a variety of products on the market in a variety of formats, such as rattle (aerosol) cans. Some of the primers claim to be self-etching, meaning that they roughen the surface at the microscopic level in order to improve the adhesion of the primer. This is achieved by adding a proportion of acid to the paint, which evaporates away with the rest of the 'carrier' solution in which the paint is delivered. These acids are not easy to incorporate into paint without some other major compromise and I don't believe in them. Etching is a vital step and I think it needs to be a separate step on its own.

One thing that almost all the primers seem to have in common is that they are toxic so, in practice, they are trying to kill you. Among the names you hear are "isocyanate" (cyanide) and "zinc chromate" (chromium). These are not substances you stir into your tea. That is why it is so vital to have really good breathing aparatus and even skin protection.

The automotive world is moving towards non-toxic water-based paints. So far, the only line of products I know of for the aviation world that is water-based are the products produced by a company called Stewart Systems. These have the important added benefits of being very low odor and easy to clean up. In fact, I recently saw a demonstration arranged by Aeropicardie, who are the European Agents for Stewart Systems at a SAAC meeting. Now the SAAC meetings are held in a restaurant. Can you imagine the potential for disaster if we were using toxic smelly chemicals in a restaurant of all places. As it happened, I can vouch for the fact that there was a stronger smell off the excellent cup-cakes that were served with our tea that evening and nobody got sick either.

This feature is particularly important to me, as I am building in such a small space and I don't have space for setting up an elaborate spray booth.

The Stewarts website also mentions another important factor in primer technology, which relates to permeability. The function of a primer is to provide a substrate which is a perfect environment for a finishing coat. One of the important elements of this environment is that is easy to adhere to. For this reason, most primers are actually porous, providing lots of microscopic gaps for the finishing coat to get down into and grip. Inside an aircraft structure, we do not use a finishing coat, for weight, time and cost reasons. So the main reason for using a primer (corrosion prevention) is actually negated by having a porous primer, without the protection of a finishing coat, that does not, in fact, prevent moisture from reaching the metal underneath. Two-pot epoxy primers are an exception here as they are fully impermeable. However they score especially badly on weight, time and cost. The Stewarts product is called a "Primer Sealer". It does completely seal the meetal and is designed to be left without a finishing coat. Together with its non-toxicity and very low odor, that makes it the perfect anti-corrosion system for a homebuilt plane.

So that's it. Decision made. The plane will be fully coated internally with the Stewarts system, incorporating Ekoclean cleaner, Ekoetch etching compound (obviously) and Primer Sealer; all water based, low odor and non-toxic - and friendly to the environment as well.