Chapter 25: Priming the Fuselage


Prep Work

After all the contouring, after all the skim coating, after all the window treatments, it was finally Prime Time!  The fuselage would finally be all one color!  The first steps were to wash the entire fuselage and let it air dry.  I then masked off the canopy glass, side windows, strakes (already primed), and all bare fiberglass not yet contoured (canard and main gear).  You want to be especially careful to seal off the windows.  The tape pulled away from two corners of two different windows, and I got some primer on them.  I was able to remove the primer, but not without alot of rubbing and swearing.  It takes nearly a full day to do this prep work, so plan accordingly. 


Applying the Sacrificial Primer

As you can see in the pictures, I sprayed on the primer with the plane outside.  After rolling the plane outside, I retracted the nose gear and the speed brake.  I taped up the nose bumper and set the nose onto a foam block to elevate the nose off the ground.  I then sprayed on three coats of primer onto the top and sides.  Once dried, I chocked the main tires and sat the plane onto its engine mount into launch position so I could spray the bottom.  One thing I didn't think about was overspray.  Alot of the overspray adhered to the bottom.  No big deal.  I merely knocked the overspray dust off with a scotchbrite pad, followed up with a clean towel and tack rag.  (If I did this again, I'd mask off the bottom first.)  Then I sprayed three coats onto the bottom.  It took all day to prep and prime the fuselage, but it was well worth it.  The fuselage looked like a million bucks.




Sanding Out the Sacrificial Primer -- Top

Of course, the fuselage didn't stay one color for long.  The next step was to sand off most of the primer until you hit the high spots.  I started with the nose and worked my way aft.  I used 120 grit and the 45-degree sanding pattern.  My high spots were at the same high spots I had during contouring and skim coating.  I made mental notes of where my high spots were before priming.  So, as soon as I saw the white micro or skim-coated glass coming into view, I STOPPED sanding.  Even at 120 grit, the primer disappeared in a hurry.  The "swooshes" gave me hell, though!  By "swooshes", I mean the cosmetic line formed at the intersection of the cosmetic strake fairing and the canopy.  I've marked off the swoosh with blue tape in the pictures below.  The swoosh line is supposed to follow a line from the canopy glass, to under the side windows, to the start of the cowl cylinder blisters.  Any inconsistency in this line sticks out like a sore thumb.  And I had two sore thumbs!  As it turns out, I didn't do a very good job of carving the strake fairings.  I thought I had fixed the swooshes during contouring, but primer does not lie!  So I had to fill in some spots and recontour several times to build up that edge into something smooth and sexy.  Perseverance paid off in the end.


I've posted a few pictures here.  At this stage, most people don't post pictures because quite frankly, they're BORING!  Each picture looks the same.  Primer, white spots, and blotches.  Like a plucked duck.  It's hard to learn anything from Chapter 25 pictures because they just don't convey details and scale.  Your hand can feel waviness long before your eye can detect it.  But, let me point out a few things.  In the first picture, you can see the white and brown spots over the canard where it joins the canard cover.  This is evidence that I had a bump there to start with.  The micro was just a tad too high.  You can also see where I'm almost to the bare fiberglass where the rear edge of the canard cover meets the F28.  I didn't round that corner off well enough when I carved the fuselage way back in....CHAPTER 7! :-)  In the third picture, note the dark spot under the forward side window at the aft corner of the canopy deck.  That's a high spot caused by the fact that the canopy deck wants to expand during the summer.  Also note the side areas between the canopy glass and the forward side window.  There's a natural tendency to contour these areas slight rounded.  Well, that screws with the swoosh line.  So I worked extra hard at flattening out those areas (both sides).  I was down to using a flat stick.  Also note in the canopy pictures that the gigh spots are generally where the TB1 bulkhead is placed.  In the last picture, you see that dark spot in the middle of the cosmetic fairing just aft of the aft window?  That's the very slight bump caused by my hidden Princeton fuel probe that's inserted into the strake.  I need to measure where that spot is in case I ever need to go back and remove/fix the probes.  Now grant it, I'm being very hard on myself because except for the swooshes, your eye could not detect these things in the primer.  It was only after I started to sand out the primer that these cosmetic inconsistencies started to appear.  Like I keep saying, primer doesn't lie.  After I spray the final primer, your hand and your eye will never detect them again.


Oh, one last thing.  You see the duct tape on the windows?  There's black acrylic tape and plastic under that duct tape.  DON'T EVER leave duct tape taped directly to the windows.  The glue will leave the duct tape and permanently adhere to windows.  The only way to remove the glue will be to use solvent.




Sanding Out the Sacrificial Primer -- Bottom


Here are some pictures of the fuselage bottom.  The high spots are on the corners as you'd normally expect.  See those two wide white stripes adjacent to the sides of the nose gear cover?  Well, those are mistakes.  I forgot that I had gently curved the micro to meet the gear cover!  I had the nose gear extended when I sanded out the primer.  I thought I had a high spot, so I kept sanding across the opening until it was all flat!  (Duh!)  My error became obvious when I retracted the gear and noticed the slight mismatch.  Notice the big white patch and bare glass in the third picture?  That was a high spot that went unnoticed before primer.  Like I keep saying, primer shows you the error of your ways.  What appears/feels flat during micro contouring doesn't necessarily mean flat once the primer is sprayed.  Primer definitely shows you where your high and low spots are.  See the picture with the mixing stick and duct tape?  They're holding the flox down in the holes redrilled for the port seat belt attachment bolts.  I contoured and primed the lower port longeron without the step or the bolts in place. 



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