[prkaye]

Somebody mentioned that they prime "mating surfaces". I'm guessing this is where two peices of metal contact each other (e.g. skin riveted to ribs). I guess priming these is more important because of dissimilar metal corrision, and also becuase the alcad layer can become chipped or cracked in these places.
Question - when priming mating surfaces, should one prime the around the area of contact, after the parts have been mated (rivited together), or should one prime the surfaces that will contact each other (e.g. the surface of the skin that will sit agains the ribs), so that there is a layer of primer between the two surfaces that are joined?
What about rivets? Should the rivits and rivet holes be primed prior to riveting, or just prime over them after riveting?

 

[dan]

"Priming the mating surfaces" refers to coating the surfaces that actually touch each other. For example, rib flange to spar web, rib flange to skin, bulkhead flange to skin. That sort of thing.

 

[apatti]

My understanding is that the reason for priming mating surfaces is at least partially because this is where water is more likely to get trapped and sit for extended periods of time.

I usually scuff the rivet lines before dimpling because it is easier without the bumps. I also prime at this time. Then, after dimpling and before riveting I shoot the rivet lines one more time. I don't prime the rivets. They are anodized. Certainly, there are other techniques... As always, YMMV.

 

[hevansrv7a]

Just a small additional note

My QB was primed extensively, but following the advice of my betters, I only prime mating surfaces that I build, such as the empennage. That said, I usually only prime one side - whichever is most convenient. It seems heavy and of no extra value to prime both sides.
h

 

[Captain Avgas]

Ensure that "betters" know best.

My QB was primed extensively, but following the advice of my betters, I only prime mating surfaces that I build, such as the empennage. That said, I usually only prime one side - whichever is most convenient. It seems heavy and of no extra value to prime both sides.
h

If you were rivetting two pieces of raw steel into an overlap and you primed just one surface...the other surface would oxidise (rust) when moisture was trapped in between. That would be obvious. I can't imagine why you would think it would be any different with aluminium.

Much of the comment on priming people get through email forums tends to reflect builder impatience rather than reflecting good scientific knowledge of coatings technology. When you follow the advice of your "betters" it might be better to ensure that the "betters" do in fact know best.

 

[hevansrv7a]

Dear Bob

If you were rivetting two pieces of raw steel into an overlap and you primed just one surface...the other surface would oxidise (rust) when moisture was trapped in between. That would be obvious. I can't imagine why you would think it would be any different with aluminium.

Much of the comment on priming people get through email forums tends to reflect builder impatience rather than reflecting good scientific knowledge of coatings technology. When you follow the advice of your "betters" it might be better to ensure that the "betters" do in fact know best.

Bob, I'll just grin and accept your comment as if you were being nice. I accept responsibility for how I use advice from all, betters included. You can just criticize me and that will be enough. An open exchange of ideas is a good thing. I don't claim to know it all. Let's just deal with facts and logic and skip the ad hominem stuff, please.

I don't think the ferrous analogy is completely right, but I'm not an expert. It's not the water I'm trying to deal with, just the electrolysis that water can promote. If there is a layer of paint between the layers, then electrolysis cannot be. If your logic were entirely right, I'd feel compelled to paint all interior surfaces, not only the mating ones. This thread was about painting only mating surfaces. I am not arguing against priming all internal surfaces, but that's another subject and there are well known pro's and con's.

My factory airplane is 32 years old and has flown over 4,000 hours. It is not primed internally and is corrosion free. That's partly because it gets periodic oil fogging internally. Also partly, it's hangared and kept reasonably dry. In the last 32 years I've worn out many steel car bodies no matter what I did to prevent rust. I humbly suggest that it's just not the same thing.
h

 

[Captain Avgas]

Electrolysis

I don't think the ferrous analogy is completely right, but I'm not an expert. It's not the water I'm trying to deal with, just the electrolysis that water can promote. If there is a layer of paint between the layers, then electrolysis cannot be.

Dear H,

It's good that you're taking this in the right spirit.

It has been said by many electrical gurus that the term electrolysis is the most confusing and misused term around. Electrolysis is a term that is very loosely applied to the corrosion process.

May I respectfully recommend that AC 43.13 (Aircraft Inspection, Repairs & Alterations) will provide a greater insight into the causes and remedies for corrosion in aircraft. I doubt however that you will find any recommendation for the priming of one surface on lap seams.

As for not priming at all. Well that depends on the environment in which the aircraft may operate and the attitude of the individual builder towards construction excellence. I'm not entering that debate because it goes no-where. I do note however that both Cessna and Piper now phosphoric acid etch, alodine and epoxy prime all interior components.....I suspect one might conclude something from that.