Priming is a hot topic when it comes to aircraft building, so we thought it warranted a separate post.
The principal metal used in the Van’s RV kit is Alclad 2024 T3, which is an alloy with a thin layer of pure aluminium on both sides. Whilst the alloy would naturally start to corrode, the aluminium layer oxidises and is reasonably corrosion resistant.
For this reason, our friends in the drier areas of the US often choose not to prime the aluminium parts, and generally they last very well. In fact, when posting on forums about priming, we often get asked ‘why are you bothering?’. Unfortunately in the UK, where we’re pretty much always going to be 70 miles or less from the sea, we have to protect the aluminium further otherwise it will begin to corrode because of the damp.
Priming isn’t a fun process, nor is it good for your health. What our inspector refers to as the ‘old school’ method involves two highly carcinogenic agents – alodine and zinc chromate, both of which are increasingly hard to source as they are known to be extremely harmful.
His words: “So if you want to stay alive, you have three options…”
1) two-pack etch primer
2) two-pack epoxy primer (which the majority of UK builders currently use)
3) newer, dual action two-pack water-based epoxy primer
After lots of reading and asking other builders, we decided to go for AeroWave 2001, which is one of the dual action two-packs. The appeal is that it is water-based, so if things go wrong it can be washed off (turns out this is very useful, see below!!), equipment is easier to clean, and the ingredients, whilst not exactly pleasant, are considerably less harmful than others on offer.
The ‘two-pack’ primer consists of the main primer itself and a curing agent. These are kept separately (hence ‘two-pack’) and mixed up only when you want to begin painting, as the mixture will quickly go off once they are combined.
We are cleaning our parts before priming with Polyfiber 310, which is a water-based alkaline cleaner recommended by other builders. It’s basically a very concentrated (and expensive!) washing-up liquid – again much less horrible ingredients than many other options we’ve read about.
We ordered our AeroWave 2001 primer from Pexa, who were very helpful and responsive. When ordering primer, it’s important to consider that it has a shelf-life (normally 12 months or less for the AeroWave). If you’re ordering a large quantity, you want to be reasonably sure that you’ll use it all up within that time, otherwise you’ll end up throwing it away and having to order more when it expires. The primer also needs to be stored at 15 degrees C or above, so we’ve had to remember to bring it inside when we’re finished in the workshop – it’s been down to -0.1C in there overnight a few times, according to the thermometer.
Once we had the vertical stabilizer parts ready to prime, J set up the compressor and paint gun, donned his protective gear and had a play one afternoon. Whilst AeroWave 2001 is ‘nicer’ than the majority of available options, it is still important to wear protective gear – breathing mask, goggles and overalls. When J opened the workshop door after testing the spray gun, I almost fell over with the smell – definitely needs a well-ventilated space, and it’s not advisable to take off your mask until it’s dissipated!
Primer is applied to the parts using a compressor and spray paint gun, which should (if you get the settings right, which takes some fiddling) give a nice even coat across the surface. J built a painting table using the Van’s delivery crate and some chicken wire, which gives a great platform on which to paint the parts without covering our workbenches in primer. It works well – the approach seems to be to do one side, leave a few hours until it’s touch-dry, then do the other one and go back later to check if it needs any extra touching up. I’m sure we’ll evolve some more sophisticated methods in due course…maybe.
We’ve been experimenting with technique and set-up, and have found a way which seems to work reasonably well. As priming needs to take place at or above 15C, we heat the workshop until it’s reached at least this temperature, then open the window at the last minute and set up a fan to blow the air away from the painting table. We have since added a filter to the fan, after we realised it was blowing paint particles about and neatly coating everything in its path in a thin layer of lime green primer. We’ve discovered that we can’t run the heater and compressor at the same time as together they draw too much current, so time is of the essence as we have to finish work, shut the window and plug the heater back in before the workshop cools down below 15C!
We have also learned through experience about equipment that needs to be handy before we start priming, such as a spare bucket to shoot excess water into from the paint gun (otherwise you’re in the less-than-ideal situation of urgently needing to clean the gun before the primer starts curing and having a reasonable quantity of water and primer mixed together with nowhere to put it!), extra paint ready to mix up if you run out, and new pairs of disposable gloves for when you get it all over your hands! I’m sure we will keep learning by doing…
We cleaned the parts thoroughly with the Polyfiber 310 solution and scuffed them with scotchbrite pads before priming, and we thought had done a reasonable job. J’s first coating showed, however, that we needed to be a bit more thorough – hilariously there were visible hand prints under the primer, where the surface had been affected by the oils from our skin and the primer hadn’t stuck as thoroughly to certain areas! Back to the drawing board…
Ghostly handprints aside, J’s first attempt at painting went reasonably well, but he realised that he’d not got enough paint on some of the parts (lighting wasn’t great when he started and it was hard to see without touching the parts). I had a go subsequently and got a more even coating, but realised that if you hold the gun in place for even a fraction too long, you end up with way too much paint and it starts to wrinkle and drip, which means it then has to be sanded back down.
The primer won’t be visible on the outside of the plane once finished, as the external paint job will cover it over, but for now the primed parts are a rather amusing shade of lime green, as it seems to be the only available colour…here are some pictures of the parts after priming:
It was on my first attempt that I found out how effectively the fan was not only dispersing the paint fumes but also the paint particles in the air. I painted the various vertical stabilizer parts and additional items including (but not limited to) the window, both pairs of spare safety goggles, the wall, the light, my hand, several paper bags containing rivets, most of the tool chest and a hammer.
Our text conversation that afternoon went something like this:
J: How’d it go?
Me: Erm. Yeah. Painted all of the things.
J: That’s awesome, all the VS parts done?
Me: Yes. But also some things I didn’t really want to paint.
J: Such as…?
Me: Window, goggles, hand, wall, light, rivet bags, tools…everything is green. Everything.
J: Ah. Glad we went for the water-based primer.
Luckily it washes off very nicely if caught before the curing agent has time to do its work. We have since fitted a filter on the back of the fan, which catches most of it and has so far avoided further objects becoming unintentionally green!
Watch this space as we continue to learn…