When sheeting a foam wing, you need to sand the mating edges of all the balsa sheets. This will assure that all the edges are perfectly straight. Then, when you glue the edges together--you are gluing a straight edge to another straight edge. Trying to glue balsa skins together without sanding them will lead to wrinkles and then bumps and warps in the finished product.

I got a piece of 3" aluminum angle at least 1 meter long. Secure it to the workbench and use sandpaper with adhesvie back along the vertical edge. Now, you lay the balsa sheet flat and slowly and carefully sand it back and forth a few strokes untill the entire edge is perfectly straight. Do this on BOTH side of EVERY sheet.

I like to pre-fab all my skins. It usually takes 4 or 5 pieces of balsa for each side of each wing panel. So, get 5 sheets and lay them out flat on the bench. Place them on top of wax paper. Push each piece up to the next piece and assure that the mating surfaces are going to line up perfectly without any wrinkles or warps.

Now use masking tape to secure all the sheets together. Just use a piece of masking tape along the glue joint between all the sheets of balsa. If your using 5 sheets of balsa, then you'll have 4 joints to tape. Run a strip of tape down each joint and tape all the sheets together to make 1 large skin.

Now pick it up and flip it over. Tape on the bottom.

Now you just slide the sheets over to the edge of the table and let the joint flex open a little bit. Run a bead of glue into each joint. You can do 2 joints and then spin the sheet around and do the other 2 joints.

I like to use regular carpenters wood glue for glueing the edges together. After you put glue on all the joints, slide the skin back on the bench and allow any excess glue to squeeze out. Wipe it off with a wet paper towel. Allow the glue joints to dry for several hours.

Paint images attached.