After glassing the wing (top AND bottom
), doing some detail work on the trailing edge, and giving the entire wing a coat of thinned resin, I cut off the ailerons (which I had put in place with hinges, without glue), cleaned up the leading edge and sides of the ailerons, and started sanding.
I dropped all the way down to 80 grit paper on my super-long block to do most of the hard work. Still, I had to put in some extra time around the center section, leading edges, and trailing edges at both root and tip to get them as nice and flat as I liked. Terry calls for that post-glass coat of epoxy, and while it does fill up the cloth nicely and probably strengthen it as well, I ended up sanding almost all of it back off. Sanding primer or spackle would have been a whole lot easier though. I think I'll move straight to primer on the tail surfaces after glassing them.
Then came the fun part... primer. I did a first coat over the entire wing, sanded most of it off, a second entire coat which got me close over most of the wing, and then repeated spot spray and sanding over areas that needed attention. For sanding I primarily used a smaller block with 400 grit and wet sanded. I even found one or two holes I sanded through the glass on the top of the wing once the primer was on... totally invisible prior to that!
Most of the wing went quickly, but there were a handful of spots that required repeated spot applications. For instance, at the corner of one of the aileron cutouts, my Xacto saw had dipped ever so slightly into the trailing edge (the part the hinges go into) that I filled back out with primer. Also near the root at the trailing edge, I needed a few coats to smooth out a small ripple left by my resin coat. And while I was at it, I also filled in around the top center fiberglass reinforcement joint a bit better. It'll all be inside the fuselage once the wing is mounted, but I want a show finish inside and out.
During sanding, I take utmost care to make sure I end up with a level finish... this means always using a block, never putting too much localized spot pressure, and always using long, smooth flowing sanding strokes, both parallel and perpendicular to the span. It was not unlike my initial shaping of the wing leading and trailing edges while sanding the wood. You have to be careful not to sand in depressions or flat spots, especially on the top curve of the airfoil and around the leading edge, where you have the smallest radii (= most risk of sanding through). I like to get the wing wet with a sheet of water and sight down the wing for smoothness.
As an (almost) final step, I put on a sheet of water on the wing and sighted down from one tip to the other, top and bottom. That sheet of water represents your final paint coat and gives you a great indication of how level your paint is going to look at the end of the day. On one of the bottom panels, I was able to detect some subtle low frequency ripples this way ... e.g. ripples that span several inches. They couldn't be more than a thousandth of an inch up and down, but your eye can pick them up in the reflection of other objects off the wing. I got out my medium long block, about a foot long, with 600 grit and used very slow, deliberate, orbital sanding motions with the block parallel to the span over the entire wing top and bottom.
At this point, the entire wing looks like glass... even the primer is shining! The little ripple disappeared. And the triple layers of glass on the center joint, top and bottom, are totally invisible.
I took it out in direct sun the next day to do a final visual check, and wouldn't you know I still found one or two tiny tiny spot depressions... including that same spot near the root near the trailing edge!! I spot-primed them. So now I have to sand those areas, re-inspect, and do another final pass with 600 grit. But it's getting very close.
When this is done, I should have a great surface for painting... which will be the last big challenge on this one. I'll move on to the tail and fuselage next though.