Thank you very much for the advice guys.
Just one more question, to prime in black on the frame. Yes or no?
Oh wait, something else. I’m also working on the purity seals. I was advised to use screamer pink on the seal for that ‘waxy’ effect. What else should I be using on this?
Depends what colour works for your minis. I think red, purple or pink may work for you well enough.
You can actually a shiny, waxy finish by using enough nun oil/agrax earthshade, or just by using the gloss variants. Just base colour, wash and a fine, subtle edge highlight (only the top of the seal, where it would naturally reflect light...same as metallic armour).
As for the undercoat, I'd go for a white undercoat before going back to it with Celestra Grey. It'll provide a far smoother finish and require fewer coats to achieve a nice effect. White over black is just hard work that you can probably avoid.