For some reason known only to Vans, there are a few places in the flaps where shims are required to fill gaps between the skins and adjacent substructure. One place I can understand – a gap formed by the overlap of the upper skin on the lower skin at the outboard and inboard ribs. The upper skin actually wraps around the trailing edge and is riveted to the J-channel formed in the lower skin. Here’s a picture of an outboard rib…look closely and you’ll see an 0.025″ shim under the lower aft side of the rib.
You’ll also see that the skin doesn’t fit well. That’s a topic for a future post.