The Priming Experiment

There are so many different primers on the market that it is hard to choose which one is best.  So I decided to set up a little experiment (in addition to my article on priming).

I have primed (or not) various bits of scrap aluminium (from Vans Aircraft) to the same standard as I have on my aircraft build.  I screwed them to a block of wood so that the metal suffers maximum moisture exposure from the woods water retention and left the whole piece on the roof of my green house at the mercy of the elements.

Every 3 months I plan to take a photo so that I can track the progress of corrosion over a long period of time.

Experiment Start: 1st August 2015

The Vans Aircraft Priming Experiment

1st November 2015 (3 months later)

Vans Aircraft RV priming

Priming 101 (UK Edition)

Priming wars!  That’s how it is known in the Vans Aircraft RV builders fraternity and for very good reason.  Being a newbe, my first mistake was to assume that there was a single primer that did it all and all I had to do was find ‘the one’!  I was SO wrong and it wasn’t until I had finally overcome my preconception that I was able to move on and fully understand priming!

What I did learn, though, is that there are so MANY different ways to prime the pride and joy that it quickly sparks fierce debate if you ask any other builders opinions.  I am certain that everyone of us builders wants to do it the right way, but with so much choice it’s hard knowing which is the right way.

Vans Aircraft priming etch primer epoxy

To help other newbe builders and to clear my head, I am writing my musings on what I’ve learn’t during my time researching, building and, finally, priming my RV-7.

So first I needed to get the most fundamental question…  What is priming?

Priming is the act of covering metal to delay or prevent corrosion.  In other words, stopping rust destroying our precious metal aircraft and, hopefully, extended it’s life.  Understanding the priming process, well that’s an art!  To help me decide I broke it down in to a series of choices. These are choices I had to make before buying the equipment and primer.  If it helps, here are the questions I asked myself…

Choice Number 1:  To prime or not to prime, that is the question…

Bare metal is very susceptible to corrosion from such things as humidity, salt air, chemical spillage, fuel spillage and so on.  The best method of protection from corrosion is to cover the metal with some kind of barrier that will act as a sealant against these risks.

Do you want to protect your metal from these risks?

  • No?  You’re done, get building!
  • Yes? Go to choice number 2…

Choice Number 2: Is Alclad protection enough?

Most (but not all) Aluminium that Vans Aircraft use is Alclad.  Alclad is a very, very thin layer of pure Aluminium that is bonded to the aluminium alloy below and is supposed to prevent the air or other evil corrosion from reaching the previous metal.  I’ve heard a rumour that Vans Aircraft did an experiment and left a piece of Alclad Aluminium outside.  After 3 years the unprimed, Alclad only, piece of Aluminium showed signs of corrosion and pitting.

Do you think Alclad Aluminium is sufficient protection for your aircraft?

  • Yes? You’re almost done, get building but watch out for all the non-Alclad parts that need priming!
  • No? Go to choice number 3…

Choice Number 3: Surface preparation…

So you’ve decided to join the ranks of the primers, welcome aboard!  It wasn’t so difficult getting here was it? No doubt you’ve probably already got your spray gun out and ready to shoot some primer.  Well hold on there tiger and remember the 5 P’s… Proper Preparation Prevents Poor Priming!

A greasy surface causes the primer to ‘miss’ areas of the metal as the grease acts as a sort of barrier.  You can see areas in this picture where the primer was prevented from working properly due to poor preparation.

Vans Aircraft RV Priming Grease

Grease is often invisible.  Honestly it is really, REALLY invisible and comes mostly from our grubby little fingers.  The only way to see this invisible grease is to paint some primer and watch it peel off or not adhere, there is your grease!

There are many ways to prepare the surface for priming such as:

  1. Scuffing the entire surface of the aluminium to a dull matt finish (excluding skins, they only need a light scratch).  I did this AFTER drilling & edging but BEFORE dimpling & fluting so that I got every mm of metal possible.
  2. Air blowing the aluminium dust off the metal
  3. Washing in Fairy Liquid (or Dawn Liquid for our US cousins)
  4. Degreasing with Paint Thinners & green scotch bright pad
  5. Degreasing with MEK (Methyl Ethyl Ketone) and a green scotch bright pad
  6. Degreasing with Deoxidine 624 (or Alumiprep, US brand) & green scotch bright pad

How can you tell you’ve cleaned properly?  Run water over the metal and if it ‘globs’ together like molten lead, it’s not clean, go back and scrub again.  If it runs of in a uniform flat flow (sheets off) then it is clean, nice job!  Make sure you have gloves on from now on to prevent the metal from getting greasy again.

FWIW I personally chose to only do options 1, 2, 3 and 4 above.

So, back to the choices: Do you want to clean the surface?

  • No? Good luck and send me some pictures of your invisible grease please!
  • Yes? Move on to choice number 4…

Choice Number 4: Adhesion layer…

This is where I found things MOST confusing.

This section is referred to by many as priming.  But the word ‘primer’ used here and again in the next section refers to two VERY different tasks.  So, for simplicity I refer to this section as ‘adhesion primer’.  Adhesion primer is designed to ensure that the next layer of primer has a really really good surface to bond with and there are 2 popular options…

Option A: Alocrom 1200 (UK name) or Alodine (US name).  This adhesion primer chemically converts the metal surface to add Chromate to the aluminium.  It is usually applied by brushing and leaving it on for a minute or two before rinsing with water (de-ionised if your really going for it).  It is also possible to dip the parts as well but I haven’t tried that yet.

Option B: Etch Primer (AKA: Wash Primer, Acid Etch or PR30B).  Usually the type of Etch Primer preferred is a Zinc Chromate Etch ‘Adhesion’ Primer.  Which, similar to Option A, adds a Chromate coating to the metal by using acid to etch the surface of the metal and bond the Zinc Chromate coating before the acid evaporates.

Never do Options A & B together, it is either one OR the other.

Almost all ‘Adhesion primers’ do NOT protect well against corrosion alone.  They are often porous to aid bonding and, if you read the technical documents, almost always require a sealing coat (see Choice Number 5 below).  So what’s the point?  Well if you want the next stage to stick like the proverbial to the metal then this layer is what’s needed.  Also being another layer, it will add another layer of protection which will help somewhat.

Do you want ultimate bonding of the next layer?

  • No? Skip this step and move on to choice number 5…
  • Yes? Wise choice, some say.  Read on…

Choice Number 5: Priming, at last!..

Finally after so many decisions, and probably much indecision, you have at last arrived!  This is what it has all been about, protecting your hard work from the evil of corrosion.

‘Primer 2’ is the top coat of primer and commonly referred to a sealer primer.  This is a layer that will seal your metal from the elements and protect it for years and years and years and years. Some say that ‘Adhesion Primer’ is all that’s needed and sure enough that layer WILL protect your aircraft for a long time because the corrosion will have to pass that layer before even reaching the metal.  But Sealer Primer will protect it for even longer.  Sealer Primer is usually an epoxy primer (or urethane primer) that dries to form an impenetrable barrier between the elements and the aluminium.

Different epoxy primers have differing properties.  Some are great fillers, but they are thick and heavy.  Some are excellent at protecting against more than just moisture, they also protect against oil, fuel and other chemical spills but they are expensive.  What you choose here is entirely up to you and some common brands used by uk builders are PPG PR143 and Aerowave 2001.

Are you going to sealer primer?

  • No? Too heavy and a waste of time. Get back to building and stop time wasting!
  • Yes? Great, now to choice number 7…

Choice Number 6: Priming everything or not…

Some builders choose to be selective in their priming.  For example they will ‘Adhesion Primer’ everything structural (ribs, spars, stiffeners and inside of the skins) only.  That makes sense, if the horizontal stabilizer is a near sealed unit doesn’t that act as another layer in the same way as ‘Sealer Primer’?

However, on the visible parts (ie cockpit spars, seat backs & cabin area) priming with both adhesion and sealer primer is a good idea as these are most likely to suffer spills, scratches, wear and tear.

And then there’s the inside of the skins.  Some people don’t prime the inside of the skins because they rely on the Alclad to protect the skin.  I decided to prime the inside skins because I live in the UK and it’s wet here, very wet!  Another reason is that if I can’t afford hanger space for a short time then I would like to have the best protection while it sits outside.

Are you going to selectively prime?

  • Yes? Good on you, get building!
  • No? Priming everything? Woah way to go man!

Choice Number 7: Bluing – The blue vinyl that covers the metal…

The aluminium that comes shipped from Vans Aircraft is covered in blue vinyl.  Many builders leave the blue vinyl on as long as possible and on the outer skins often until they have finished building which could be many years later.

Vans Aircraft, however, recommend in the build manual to remove the blue plastic as soon after inventory as possible.

A very wise aircraft painter told me that he has seen many RV parts that have been in long term storage with the bluing left on suffering from corrosion because the bluing creates a moisture trap.

Other RV builder say that the bluing protects from scratches.  But the bluing isn’t very thick and any scratches that the blue vinyl can protect from can easily be buffed out with scotch bright.  Any scratch deep enough to tear the vinyl would have gone through it any way.  So in essence it’s not really protecting from much.

I decided to leave the bluing on my metal until I started building the part and then I removed it completely, even from the skins.   It gets more difficult with time to remove the blue vinyl so choosing the right time to remove it could save a lot of work.

To protect the skins from corrosion during storage I am priming the outside of the skins the same way I am priming everything else.

Removing all the blue vinyl?

  • Yes? Good on you, don’t forget the last step!
  • No? Read up on using a soldering iron to help remove the vinyl, it really works!

Choice Number 8: Jointing compound…

Now that everything is ready to be put together and has been adhesion and/or sealer primed (or not) the last important corrosion related question is jointing compound.

This compound sits between any two metals joins such as spar to rib and/or spar to skin.  The purpose of the jointing compound is to repel moisture from between the joints.  Moisture that sits between the joints could remain trapped allowing it more time to attack the surface.  Of course you could leave loose rivets which would allow the moisture to evaporate but that’s probably not a good idea.  Instead a jointing compound should protect the joints.

Where jointing compound is REQUIRED is between dissimilar metals.  It will stop the metals reacting and greatly reduce corrosion in those areas.  Some builders use jointing compound on every joint and even on primed parts.  My aircraft painter suggested that primed parts already had 2 layers of protection and that jointing compound was not necessary.  My inspector insisted that jointing compound IS absolutely necessary.  I love self building!

Are you going to use jointing compound?

  • Yes? Welcome to the world of budgie poo!  Let’s build!
  • No? Read on…


Remember that priming your aircraft is an intensely personal choice which is why it always invokes such fierce debate.  There is no right or wrong here, only YOUR choice (actually I think the LAA insist that aircraft built in the UK are primed).  All of these steps are optional and results will vary depending on your ability, the chemicals, temperature, tools, the environment, weather and so on.  It is even possible with some primers to simply spray epoxy primers (primer 2) directly onto the bare metal skipping all the other steps!  It’s your choice.

With all that said and having ranted on for hours about priming I am about to contradict myself enormously…

Many aircraft manufacturers don’t prime at all!  

WHAT?!  That’s right, I’ve looked.  Next time you go flying have a look inside the aircraft your renting.  Likelihood is it won’t have been primed but yet it’s not rotten.  How can that be?

One reason is because owners regularly spray ACF 50.  ACF 50 is essentially a grease that creates a barrier of protection instead of all this priming nonsense.  Another reason many aircraft survive decades without primer is because they are well maintained.  They are kept in well ventilated hangers, have never even heard of IFR, and are loved more than the family pet!

Of the aircraft that I have seen with corrosion it is usually on the outer skins and caused by scratches, stone chips or acidic bird droppings that are left.  Sometimes even a poorly painted aircraft will suffer corrosion if the paint does not bond correctly causing a moisture trap between it and the metal.  But what often amazes me is the spars and ribs show no sign of corrosion whatsoever.

So it seems that corrosion may not be the evil nemesis after all. It might be the way we treat our aircraft AFTER we’re done building it.  So the final and MOST IMPORTANT question should be…

Choice number 9: How do you plan to treat your aircraft after it’s built?

*Disclaimer: This article is written to the best of my (little) knowledge(!) and likely to contain many errors and offend many people.  Please feel free to comment, correct me, flame me and insult me as appropriate after all we're RV friends!
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