[RV7Ron]

I have a question for the guys that are priming interior parts. Here is a situation that I'm sure I'm not the only one that is experiencing and I need some guidance from some wise old veterans.

I match drill parts, disassemble and prime with a two part epoxy. Cleco it back together and get ready to set rivets...but low and behold the primer thickness has now closed up my hole slightly and I cant get a rivet into the hole. No problem, grab a drill and ream the hole. I have now removed my primer in the hole. Should I re-primer this before setting a rivet or am I good to go skipping that? Can I assume that the rivet hole/rivet will be a moisture-free environment or is this going to corrode later? This has been bugging me and I didnt find anything in AC43-13 referencing this specifically.

 

[JonJay]

I would not worry about it.

That is one thick coat of primer. I can not imagine that leaving the hole unprimed will ever causing an issue. However, by using a two part epoxy primer, it tells me that you feel the highest degree of protection is necessary so what is stopping you from continuing that thought process and priming the hole?
If you top coat, for the most part, the top of the rivet will be sealed. The only place for moisture to enter would be through the shop head. Keep in mind, moisture has to be present for long enough to allow electrolosys to occur or corrosion wont start.
My Fiat G.46 projects have quite a bit of corrosion in places. There are areas the some corrosion is evident on the skin and the shop head of the rivet. However, when you drill out the rivet, the hole is perfectly preserved, as well as the area under the shop head. I have to believe it is a space that allows little moisture to be retained if any.

 

[WingsOnWheels]

Just throw the rivet in and move on. Unless you are planning on parking on an aircraft carrier, it won't be an issue. The rivet seals the hole up pretty tight.

 

[terrye]

Corrosion in rivet holes

I am using a 2 part expoy primer and have occasionally run across this too. Unless the primer is really thick (too thick according to the application literature) this shouldn't be a problem on dimpled holes (AN426 rivets). These holes are quite a bit oversize due to the dimpling process. Drilling these out to #30 or #40 should still leave some primer thickness. Through holes (AN470 rivets) will be the drilled/reamed size and only a bit larger than the rivet. You might try drilling these out to 3/32" or 1/8" and see if you can get the rivet in. If not drill to #30 or #40.

In many cases I have touched up the rivet heads with epoxy primer and a Q-tip after driving them, but never the holes. I think rivets are pretty much gas/liquid tight after they have been driven.

And in the end, there are lots of rivet shop heads that cannot be practically airbrushed/Q-tipped once they are installed. The entire wing bottom skin rivets are in this category and these are probably more susceptible to corrosion than the top skin rivets.

I think the most important parts to prime are the non-alclad parts first, the inside skins/ribs/bulkheads second, and finally the rivet holes.

You might experiment with the paint processes to avoid closing up the holes as well. Pay special attention to mixing, dwell time, filter the paint as you pour it into the gun, etc. Doesn't take much epoxy thickness to protect the part.

 

[fehdxl]

check

I'm thinking along the same lines as everyone else. BTW, as a way to save time, I prime first, then match-drill, de-bur, dimple, and final rivet. I too was tired of running the reamer through the hole just to remove a bit of primer. -Jim

 

[rolf]

Drive them Wet

In A&P school (4 decades ago) we drove the rivets wet, after dipping in Zinc-chromate primer. We all had Green or Yellow fingers for a week!

 

[GAHco]

I did it my way ///||\\\

The 2 parts epoxy, in my opinion goes on after riveting.

The rivets are anodized for corrosion resistance. In most instances it takes two to tango, If your sheet was clean and you will be priming over the driven rivets you are almost certainly in good shape.

I primed all of the interior structure with ZC wash primer and then riveted.

The outside will be primed with ZC wash primer and Gray Epoxy, both after the exterior panels are riveted in place.

 

[Rick6a]

Ron,

As an Akzo epoxy primer fan, I am aware of the problem you sometimes face, but in my experience the problem has only occurred in larger diameter bolt holes. Similar to past Piper Comanche production, I routinely prime individual parts prior to assembly. As you probably know, even though not fully cured, Akzo dries to workability in about an hour. Epoxy primer is tough stuff and overspray that manages to find its way into a bolt hole, especially a close tolerance bolt hole can later make it difficult or even impossible to insert a fastener. If you are experiencing problems inserting solid rivets into dimpled holes, I suspect the cause may be the application of too much primer.

Primer buildup (or other form of contamination) can occur in holes during the assembly process and is not at all unusual. The fix is to clean the hole. That said, priming bare fastener holes may be unnecessary. I base my opinion on this: Even though my employer implemented the finest corrosion control techniques known to man, priming bare fastener holes was almost never required and that included new production on the mostly aluminum T-45 Goshawk (assembled with AD5 and AD6 rivets like we use AD3 and AD4 rivets) and the even more sophisticated F/A-18 Hornet salt water combat aircraft. Other than a required dab of proseal under the manufactured head of the rivets, if we were supposed to prime all those holes too....we didn't get the memo.

 

[RV7Ron]

Thanks guys, just the sanity check I was looking for. It doesnt happen all the time, maybe I'm puttin' 'er on a little thick at times? I thought it was a non-issue but I wasnt positive, now I can get some sleep!