On a couple rallye wheels I cleaned up I used EvapoRust down in the seam area. Before spraying the wheels with epoxy primer I poured a little reduced primer in the seam and rolled it around to help seal it.

On a wheel like that you can pack modeling clay in the open slots from behind and then pour a little Evaporust in the seams. The clay should seal it long enough for the Evaporust to work. Might want to put the wheel on some newspaper or plastic sheeting just in case you get a little seepage.

If you take steel wheels to have them blasted and powder coated here you'll get back a wheel with media bits packed into the seams and very little coating in the joint. First couple times the wheel gets wet you'll have rust starting. Expensive and no attention to detail so I just do them myself with good primer and paint.