What I did to my crossmember: Clean. Blast. Primer. Paint(2k). Cavity sealer on the inside. I used a grease based sealer which has to be heated before applying it.
I have a C20 and my control arms look exactly like the ones in your pictures. The upper control arms are identical except the bushings and ball joints, the lower ones have different stamping for the shaft with the nuts. Try If the ball joints fit into your spindles.