Update on the prime corrosion experiment.
See:
Primer / corrosion experiment
Primer / corrosion experiment [after 1 month]
Primer / corrosion experiment [after 3 months]
The samples remained outside for 20 months now.
Here is the result:
General conclusion:
A+B: corrosion after 20 months without any protection
C: Alodine without any primer offers good corrosion protection
D: Anodized aluminium without any primer offers good corrosion protection
E+F: Washprimer protects against corrosion but is not very durable without a top coat and cannot handle solvents
M+N: Epoxy primer is better than washprimer but dries longer and is thicker (adds more weight)
G+O: Alodine is perfect for primer adhesion
H+I: Washprimer does not stick to anodized aluminium
P+Q: Epoxy primer sticks to anodized aluminum
Other: With a 2K top coat the results are equal, independent of the preparation
For the Sling my conclusion:
For the interior not in contact with fuel or oil: 2K wash primer (light, dry in 1-2 hours)
For the interior in contact with fuel (part of the wings) or easily scratched (center fuselage): 2K epoxy primer
For the anodized wing spars: 2K epoxy primer
For parts that are difficult to paint like the trimtab: submerge in Alodine