Priming rear spars, countersinking main spars

I had really wanted to get the rear spar stuff primed today…so I was pretty disappointed when the forecast weekend rain turned out to be a miniature monsoon this morning.  I was kind of bummed, even though there’s plenty of other work to do.  But then the rain cleared up and it looked like I’d have the whole afternoon rain-free!  So when I went out to the garage, I stuck with Plan A and went to work on the rear spar components.

Everything was basically ready to go, except that I needed to dimple the flange portion of the spar reinforcements and countersink the outboard row of web holes.  Since those holes will be covered by the outboard aileron hinge bracket, they need to be flush.  The countersinking was mostly uneventful, except that one of the holes is pretty close to the flange, close enough that the countersink cage interferes.  So I fired up the bench grinder and “customized” the cage.  No problem.

Now it goes in tight spaces!

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With the countersinking done, I proceeded to dimple the flanges of the reinforcements.  Things were a little tight to dimple with the hand squeezer, but I got it done, without problems…or so I thought.  On closer inspection, it turned out that I’d bent the flanges a bit on the reinforcements, such that they didn’t want to sit flush with the spar channel any more.  So I had to get out the hand seamers and bend the flanges back into place.  I really should go ahead and get an Avery vise-grip dimpler for tight jobs like this.

With that, I set about scuffing all the parts for priming.  I did the spar channels first, and wow…I was not prepared for the amount of work that took.  Up until now I’ve been mostly scuffing pretty small parts.  I went through two Scotchbrite pads just on the channels!  Once everything was scuffed and then cleaned thoroughly, I set them out to dry and took a break inside for a few.

Meanwhile, I needed some work to do in between shooting coats of primer.  If I move forward past doing the rear spar subassemblies, the next thing is to start cleaning up and straightening the ribs.  Or I could start looking at some of the main spar work.  Since I’d already been countersinking, I decided to stick with that and work on the main spar.  I’m still waiting on the backordered nutplates for the tank attach screws, but there was no reason I couldn’t get going on the inspection cover nutplates.

This was another good example of the new minimal instructions.  They just say something like “Countersink the main spar for the nutplates.”  They don’t mention that in addition to countersinking for the covers, the nutplate attach rivets have to be flush too.  So there’s countersinking before you can even get tot he countersinking!

There seem to be some different ways that builders have done this task.  The specific problem is that the countersink cutter needs a hole deeper than the thickness of the spar flange to keep it centered.  Some guys will use drilled pieces of wood for this, but the instructions simply note that a #40 countersink will center sufficiently in the #6 nutplate.  I just went with the Van’s method; I final-drilled the rivet and screw holes, countersunk for the rivet holes, and riveted the nutplates in place.  Then I came back and countersunk for the actual cover plate screws.

Countersinking the rivet holes (I used a block of wood to help guide the countersink cutter):

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And the finished product, nutplates in place and cover countersinks done:

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It was somewhere in the middle of this that I made an unpleasant discovery.  See, I primed the spar channels first, then set them aside and worked on the other pieces.  When I finished work on the first main spar, and went to put it by the primed spar channel, I found this:

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Not sure what went wrong here.  I scrubbed the spars thoroughly, and was 99% sure they were good and dry before spraying, but maybe I was wrong about that last one.  So I guess I’ll be sanding these down and starting over again.  It’s not really difficult work, but it kind of annoys me since priming was the #1 thing I wanted to get done today.  Tomorrow looks very rainy, and I’ll be out of town next weekend.  But hey, there’s plenty of other stuff to work on…

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