With the plane upside down and wing reattached, I began by cutting the rest of my foam pieces to shape. It was during this process I realized that, during my fuel tank construction, I glued the bottom center section of foam incorrectly. By incorrectly, I mean that the foam was actually recessed below the airfoil shape for about half a square foot. I had problems fixing similar trouble spots before, but I've decided to try a different method with this area. I didn't take a before pic, but the after pic gives the general idea. The darker area is the low spot.
I decided to go ahead and sand the center section to the airfoil shape so I would have a solid understanding of where the foam was low. After that was accomplished, I traced and cut out a piece of foam 3/8" thick that was 3" smaller around the edges than the space that was low, and I glued that on. My next step will be to sand that to shape, then use micro to fill the outer band of the low spot, utilizing a straightedge while the micro is still wet to get a nice flat surface. This should work much better than my previous attempts, which were to glue foam over all of the low spot and sand it away. The problem was that the micro didn't sand as readily as the foam, and created high spots.
With that figured out, I turned my attention back to adding foam over the rest of the bottom of the wing. Pretty straightforward, just make sure there's enough foam sticking up to sand away, and enough foam left after sanding that it won't flex after fiberglassing.
I came up with a simple (sort of) way to help hold long pieces of foam in place when they want to sag. First I cut off a piece of string about twice the length of the foam. Next I poked holes every foot or so in the foam I want to attach. The string gets secured at each end, by whatever means necessary. Finally, the string gets pushed up through each of the holes, then pulled until there's no slack. When you're finished, you've got plenty of string loops to help rearrange or secure your foam. I didn't end up using them to hold the foam in place, but they were very helpful for making small adjustments while the micro cured.
Top side:
Bottom side:
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