I fiberglass both top and bottom. This ishow I do it:

I measure out about 2 1/2" from each side of the center joint, and use a straight edge to cut away the covering. (Obviously you need to be very careful/delicate and cut through the covering, without cutting into the wood. Otherwise, you'll weaken the area just outside the area ofreinforcement work you are about to complete.)

Rather than just using one layer of heavyweight FG cloth, I use two pieces of slightly lighter weight 3" fiberglass tape. Or you can use standardcloth, cut into 3" strips. Each has its advantages and disadvantages. The tape is already cut and its edges won't unravel on you; but the woven edges of the tape tend to be thicker, can sometimestend to curl up slightly,and due to both of these factors leave a moreobvious raised line. The cloth takes an extra step to cut, and can unravel on you; but you may already have it on hand, and the edges tend to lay flatter and seem to sand out a little smoother with a little less work.

I lay one piece primarily on the left wing panel so that 2" are two the left of the wing center joint, and the remaining 1" overlaps this joint, extending about 1" onto the right wing panel. Then I lay out the piece on the right panel similarly, but with oppositeparameters. (See the primitive drawing I made for you, below.) When completed, this arrangement gives two overlapping layers where it is needed most . . . at the very center; and 1 layer extends another inch to each side where there is still significant pressure being applied by the G forces in the turns. This arrangement also makes sanding it relatively smooth much easier than just one extremely thick piece that more abruptly announces where the reinforcement ends, and therfore would make recovering this section more difficult, and far less attractive.

After curing, give a quick rough sand to remove high spots and those potentially flesh-piercing FG "needles". Fill with balsa filler, and sand relatively smooth, but don't get counterproductive by cuttingintomuch of the cloth. Sand it just well enough to get the covering to lay over it reasonably well when you re-cover this section. (Don't worry: the top is hidden by the fuse, and most people don't get a close or prolongedlook at the bottom.)

Recover with compatible piece(s) of iron-on covering (overlapping as is usual), and you are done.

Hope this helps.