I built a speaker enclosure out of MDF attaching the sides with PVA
glue and brads using butt joints and spraying the surface with
automotive primer-surfacer to get a smooth surface. The result was
very good, but there was a SLIGHT hint of the butt joint seam at the
overlap where one piece of MDF butts with the surface of the other
piece. I think there is a slight difference between the porosity of
the end vs. the surface or something between the two pieces. I let
the two pieces dry for about a week before sanding the edge flat.

I have been experimenting with different methods to hide the seam for
future projects. I have tried:

A washcoat of PVA glue covering the edge and the overlap area (first
effort not a spectacular success)
Using drywall joint compound to fill the inperfections (was not
impressed with the result)
A coating of superglue spread over the area (this seems to hold
promise)

Has any one tried an approach to stabilize the edge fibers of the cut
edge? It would appear that might hold the most promise from the
superglue result so far.

Any other thoughts?