'Wash-in' or 'wash' is a common term used in model building. Wash-in is when the angle of attack of the wing chord increases as you approach the tip of the wing. When viewing the model from the front, it looks as if the trailing edge of the wing is lower than the leading edge of the wing near the dihedral rib. Wash-in is an important adjustment that must be built into the wing. The purpose of wash-in is to compensate for the torque of the wound rubberband. The increase in angle of the wing from the washin causes more lift and counteracts the torque. Most  models only need a small amount of wash-in to fly well, and it is easy to add this to your wing.

First, take your razor blade and cut the leading edge wing spar (next to the center rib) about half way through the spar like you did for the dihedral in the wing tips. Then bend the wing upwards to crack the wood, like you did for the dihedral in the wingtips. (Note that the cut is made on the right side of the center rib as shown in the photo.)

Then lay the wing down on the table, and place a 1/8" balsa shim under the leading edge spar to hold the wash-in setting in place.

Place a couple of quarters on the wing to hold it down on the table. Then apply a drop of superglue to the crack you made in the spar by the center rib, and then apply the accelerator.  Let it sit for about one minute to dry.

Remove the weights, and then lift the wing off the table (it may be necessary to slide the razor under the spar to cut it free from the table). The wing should now have 1/8" of wash-in. (see photo below, and how the trailing edge is progressively lower than the leading edge as you move to the right.)

Now is also a good time to fill in the gaps at the dihedral breaks. Place a small drop of glue into the gap and apply accelerator to cure it. This will make the wing tips stronger, and prevent them from breaking when the model is flying and when it flies into something (it will happen!).