The best is anodize without sealing. However the stycast application has to be as soon as possible after the anodize before the anodized surface absorbs humidity from the air. This is a common procedure for painting of aluminum alloys.
However, if the plate is already anodized and sealed the surface should be cleaned using a solvent recommended by the stycast manufacturer (see their catalog and technical info.
I have had good sucess with non anodize aluminum with a 125 surface finish and potting with stycast potting to above 1100 PSI in house tests. We use this potting to seal to about 250 PSI on Teflon wires in the ocean. I think if you clean the aluminum with a deoxidizer (Protex) and any surface finish will work.