Primers

I have been painting the exterior surfaces of the aircraft with PPG DTLF/DP402LF primer and Concept Urethane. Satisfied about the Urethane but the primer is in my opinion not great:

Primer contains a lot of filler and does not crosslink as would be expected from epoxy. It does not sand well even after a few days and can be wiped of with MEK indefinitely. At $ 90 a qrt it is not cheap and I expected something better.

To compare notes I like to add that in the past (long past in South Africa) I started building this RV6 and primed all interior surfaces before riveting with an epoxy primer bought there (don't remember make or spec). This provided a sealed surface that to this day is hard and clean and cannot be removed with solvent.

Painting is just about the most frustrating aspect of this built!!

Comments/advice appreciated

Sounds like a primer for auto use. There you need good adhesion and filler and eventually corrosion resistance. For airplanes you need mechanical/chemical resistance, adhesion and eventual corrosion resistance, but no filler.

Priming for a surface coat is a very different function than priming for protecting the internals. The internals (the core metal) are already protected by the alclad.

My view on this (at the moment) is there are lots of views on priming, but so far I cannot remember I have read about a single incidence relating to a real corrosion problem on a RV.

Vans use a transparent vinyl? primer with no anti corrosion agents on their QB internals. This is probably more than good enough for 99% of all cases.

On the internals I use a epoxy for aviation use (Strontium chromate). The more I think about it, the more overkill it seems to be. But - the epoxy is tough (both mechanical and chemical), thin and it looks real cool, and I can't really do anything wrong except adding a little bit of weight. On the external I will probably only use a superthin coat of two component acid etch (also for aviation use). It has no chrome, only zinc oxide. Zinc oxide is not a full anti corrosion agent for aluminium, but it prevents the most common kind on sheet aluminium covered with paint; filliform. As top coat there will be some urethane.

I have used the PPG DP50LE (...the 50 is the gray color) epoxy primer with the DP401 catalyst and had excellent results. Very durable and scratch resistant. I did not try and wipe it down with MEK though.

There is a required "induction time" after the primer and catalyst are mixed with the DP401, which I followed. I have not used the DP402LF catalyst.

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Some additional notes:
Once the epoxy primer is applied, you have up to 1 week before the topcoat needs to be applied. You can also wait between 15 min and up to one week between coats of the epoxy primer. The completed cross linking occurs at that time.

If you wait longer than one week, then the primer needs to be cleaned, sanded, and re cleaned, before the next application. No cross linking will occur, just the mechanical bond to the undercoat.

All the times are targeted at a temperature of 70 deg F. For each rise of 15 degrees, the times are cut in half.