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prime assemblies

YellaDawg

Well Known Member
Sponsor
curious to see if anyone recommends priming completed assemblies, such as the rudder, horizontal stab etc.? why would you not do it?
 
For one you would have to know for sure which top coat you are going to use so the are compatible . Two you will have to scuff the primer before top coat.
 
Faying surfaces.

Assuming you are talking internally, then priming complete assemblies would be pretty much a waste of time. The greatest corrosion risk is between the faying surfaces, i.e. between two bits of sheet metal that overlap, such as a rib flange and corresponding skin surface, or between two overlapping skins. Capillary action draws moisture into these areas, which when evaporates, and gradually salt deposits left behind get higher and higher in concentration. The purpose of the primer is to protect these surfaces within the joints. You will see some people only prime along the rivet lines, so they are targeting the high risk corrosion areas, but saving weight by not priming the bulk of the skin. Alternatively you could fay seal or wet rivet the joints using some type of flexible joining compound so that water can't make its way between the surfaces at all. You will do this in the tanks to stop fuel leaking out, but it is also common practice all over seaplanes because of the high salt concentrations in the water. I have fay sealed my belly skins as this will be the area with highest corrosion risk on my aircraft, especially near the rear as it is a tail dragged. I am using sikaflex pro, as it is very flexible, had a long working time, and isn't nearly as toxic as our beloved proseal. If you dig a little on this forum, lots can be found about corrosion and Fay sealing.
If you are talking external priming, then the above information is probably irrelevant.
Tom.
 
More info than you will ever need

Check the Primer section in the "Neverending Discussions" part of the forums. Tons of information and techniques. Some of it good...

I of course primed all my parts before assembly which is the ONLY possible correct procedure. :p
 
I was asking the same question, I didn't get a lot of response. I am using epoxy primer so it would have to be sanded before top coat, that is the biggest down fall I can see. My skins are alodined so I would like to protect it during assembly.
 
curious to see if anyone recommends priming completed assemblies, such as the rudder, horizontal stab etc.? why would you not do it?

If you mean the exterior.... now a days most paint systems use a primer that has a time period requirement of when the primer must be top coated with paint. It is usually a very short period (24-48 hrs)
If that time is exceeded, the primer has to be sanded and then ideally the surface re-coated with more primer to assure good crosslink between it and the paint coating.
So priming way in advance of finish paint only gives you more work, more weight (from extra primer), and more expense, for very little other benefit.
 
priming

yes I was thinking of the exterior. The time requirement, I didn't even consider the time requirement. Makes sense. Thanks for the help!!
 
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