When I was a young lad, I had to suffer through years of braces and other painful torture devices in to correct an unsightly overbite. In fact, I only narrowly escaped being required to wear headgear to junior high (the horror!). So it was not without a certain sense of deja vu that I set out to correct a similar condition on my airplane cowling. It's a bit hard to see here, but the way my upper and lower cowl halves fit together causes the one to be out of alignment with the other by about an eighth of an inch:
Knowing what I know now, I could probably have corrected this early on when I was initially fitting the cowl halves together. But now that everything is trimmed and drilled, it would be impossible to shift the relative alignment of the upper and lower cowl without affecting everything else, including the parts that actually turned out pretty good. Still, no matter – even though it's a pain to work with, fiberglass can erase many sins. I first obtained some 1/8" thick closed-cell foam, and cut a half circle to fit the cowl. With a sanding block I matched the contours, then epoxied it to the lower cowl. (the notched-out area in this photo was repaired before subsequent steps}
A weighted board covered with wax paper ensured the foam would adhere evenly all around and leave a flat surface:
On top of the foam I laid up three plies of 8-ounce bid cloth. The foam is just a substrate, of course – the epoxy and glass will provide the actual strength.
After several hours of sanding and fitting, I had the excess glass ground away and the shape looking pretty good:
It took a lot of on-and-off fitting and work with the grinder before the top cowl would fit again:
Now the two halves are roughly even, but there's still room for improvement:
I stuck some packing tape to the top cowl as a release agent, and reinstalled both halves on the airplane. I then squeegeed a thin layer of microballoons and cabosil over the new layup, as well as the cowl joint behind the spinner:
After another couple hours of sanding, the surface is getting pretty smooth:
In case you were wondering, sanding fiberglass is dusty and hot and no fun at all. Not even a little bit.
Now we're talking!
Nice and flat all around:
As a bonus, the fit behind the spinner is greatly improved as well. Good enough to give to the painter for the final detail work.
This little mini project was one of those things that isn't strictly necessary to make the airplane fly, but it would have bothered me to leave it undone. I think they call that craftsmanship. I'm just glad the airplane won't have to wear headgear.