Did you paint your edges before sewing? If you have a belt sander you can use a finer grit and sand the excess off, then refinish your edges. Just remove enough so both edges are flat. Use a beveler after sanding. Then burnish the edges together.
As mentioned before when using contact cement, make sure you let both pieces air dry before pressing together. Depending on the application sometimes I use basting tape instead of cement. Doesn't hold as well but is alot easier.