Dark spots on anodised castings

A. First, I hate castings.
What comes to mind is possibly a need for a better cleaning and or desmut step. Try to not over clean or over etch. Add vigorous rinse steps between each process tank to clear some of the chemicals out of the pores in the casting. Actually, starting out the very first step in a rinse tank to fill the pores with clean water will help keep out the follow on chemicals.

A. Although Casting always behaves this way(for improper process), yours is too serious.
increase your bath temperature if below 16 °C or lower your current density if higher than 13A/ft^2.

A. Castings and porosity can't be separated so you have to tailor your anodizing, both pretreatment and post treatment, to mitigate their deleterious impact. For instance, avoid etching the casting - don't open up more pores or expand those already present. Caustic etching is especially bad in this respect. If you must, keep time to bare minimum. Following anodizing, soak the parts in a weak, mildly alkaline solution such as sodium bicarbonate to pull out some of the sulfuric electrolyte from the pores. Good luck!

A. John,

The porosity is almost certainly the source of your problem. Have you considered sealing the casting before anodising, this will reduce the problem, although looking at the casting I don't think you will ever get a great result.

The high silicon in the alloy makes it more difficult to anodise but I don't think it is the source of your problem.

If you can't seal the casting you will have to alternately rinse the casting in cold and warm (50-60C) water for several cycles to try to remove the sulfuric acid from the pores.