Preparation and Priming - Just one view
On the RV List, there seems to be one recurring subject that invokes a lot of passion and very strongly held views, and that is the subject of surface preparation and priming. I've followed those disussions with a great deal of interest, because, as I have found out through conversations with other builders and professional engineers, there is a lot of misinformation out there.
There's probably some in here, too. However, it seems to make sense to me, and I hope, after reading this, it'll make some sense to you too. At the very least, hopefully you will understand why I've made the choices that I have.
I came to the whole preparation and priming questions with a lot of confusion. I knew that there was this poisonous stuff called Alodyne, and it made aluminium a straw colour. I'd heard that the fumes from epoxy paint systems caused cancer, and that if you didn't do everything right, you'd keel over right there in your workshop, paint gun in hand.
Time for some research.
After about of month of correspondence and phone calls, and a lot of well intentioned but contradictory advice, I called the engineers at the Alpine Fighter Collection. And this is what I learned.
First of all, you need two plans. A preparation plan and a paint plan. Then stick to them. The plans you devise depend on the intended lifetime and use of your aircraft. New Zealand has extensive coastline, with lots of salt air. I hope to hangar my aircraft, but I cannot speak for the person who may buy her (if I one day decide to sell her - not likely!) Also, I want this aircraft to be around for as long as, or longer than, I am. One builder said "We're not building these aircraft to last for 50 years." - my unspoken reaction was "Speak for yourself! Sure, there may not be any fossil fuels left in 50 years time. Does that mean I shouldn't build well, and build to last?" So my preparation and priming plans have been developed with significant, long term corrosion control in mind. However, I also want to finish building the aircraft, be environmentally aware, and be safe while I'm doing it, so ease of use, the environment and my family's health factors are considerations.
There are three types of material used in building an RV. Steel, Aluminium alloy stock and AlClad. My preparation plan treats each of these differently, and assumes all drilling, shaping and deburring has been completed, apart from final assembly.
STEEL - Remove any protective covering. I use a 3M product called SoftSeal, which sprays on, forms a protective seal against moisture, and later comes right off with a kerosine soaked rag. Polish to a satin finish with 600 grit wet n' dry soaked in kerosine. Clean off with kero. On with the gloves, and degrease with Prepsol. Airblast dry and it's ready for priming. Be careful not to touch the part with bare fingers or messy gloves before you prime it.
ALUMINIUM ALLOY STOCK - Polish to a satin finish with 600 grit wet n' dry soaked in kerosine. Clean off with kero. If the part has sat for a while after this step, give it the once over with a scotchbrite pad. On with the gloves, and degrease with Prepsol. I do this in a tray of prepsol and a sponge, wiping the part thoroughly. Rinse off with water. If the water beads, then rub that area with a (different) damp sponge. Immerse the damp part in Alodyne for just one minute (I use another tray) making sure the whole part is rinsed, if not covered. Remove the part and quickly rinse with water. Don't overdo the Alodyne - the part should be a light straw colour, not a rich gold. Even ten seconds over the minute will result in a noticeably deeper colour. Air blast dry and it's ready for priming. Be careful not to touch the part with bare fingers or messy gloves before you prime it.
NOTE: Prepsol and Alodyne are poisonous. When using these products, ensure you wear gloves, eye protection, and adequate ventilation. As you rinse parts with water, make sure you minimise any discharge into stormwater systems.
ALCLAD - The preparation for Alclad is slightly different, and requires some explanation.
AlClad is an alloy with a thin layer of pure aluminium only a few thousand's of an inch thick. The surface of this layer oxidises to form a hard protective layer over the softer alloy. Normal aluminium alloy does not have this protective layer, which is why Alodyne is used to chemically convert the soft surface to a hard layer (see above).
In other words, the Alodyne process creates a tough "shell" for exposed alloy. Unscratched or unmachined AlCLad has that tough shell already. However, as you drill and file the edges of AlClad, you are exposing alloy, which will need protection (via Alodyne). However, note that the Alodyne will NOT take to the unscratched surface and non machined edges. That's fine! Those areas are already protected by the oxidised layer.
What about scratches in the AlClad surface? As mentioned in the manual, these need to be polished out with 600 grit wet n' dry. In the process, however, you will be removing the pure Aluminium layer, exposing alloy, to which the Alodyne WILL take.
In other words, if you have exposed alloy by polishing the AlClad sheet, the Alodyne will take to those areas, and they will turn gold. Where the AlClad surface is instact, the Alodyne will not take, and the colour remains unchanged. Either way, the underlying alloy is protected. What you should NOT do is methodically strip away the entire AlClad surface just in order to get a consistent Alodyne colouring. My preference is to minimise scratches to the AlClad surface, polish any scratches out as lightly as possible, and let the Alodyne look after any exposed alloy (inside drilled holes, filed edges etc).
The upshot is that my preparation plan for AlClad is the same as for Aluminium alloy stock, except that the 600 grit wet n' dry and scotchbrite are used SPARINGLY to remove any scratches and to polish exposed alloy (eg. filed edges).
Preparation is only part of the story. If I was building and flying in Arizona, I could probably get away with letting AlClad and Alodyne do their stuff. But I don't, and I still have contacting dissimilar metals to deal with. So, in accordance with my stated intent, my priming plan is simply "prime everything before assembly".
That was easy. But what primer?
Well, if I'm priming before assembly, it will have to be both bulletproof and flexible. I wanted excellent corrosion control properties, and - here's the kicker - it has to be safe to use. By that I mean no carcinogens, no chromates, and no isocynates. I should be able to spray it without looking like an extra in "Outbreak". A dual cartridge mask is okay, but a positive pressure breathing system I can't afford. Something with a good track record and an "as used by" endorsement would be nice too.
I had found an isofree Croda Lusteroid base, but it required two coats of different two pot mixes. I mentioned it to Richard at Fliteline, and he pointed out that they use Imron 830. It's a two pot epoxy, as specified by Cessna, hasn't got any nasties, and requires one coat before assembly with a light "dusting" coat over the rivets after assembly. Sounded like a plan to me! It has other components, like a specific reducer (thinner), and the all up cost for 4 litres made my eyes water (about NZ$300). I'll be using an HVLP conversion gun on my 12 cfm compressor. It'll be interesting to see how well it performs.
So my priming plan is - wipe over parts with a clean rag and prepsol. Spray Imron 830. Miller time.