x-InsigNia-x
Because CSM models are usually undercoated black and then painted red. but i dont know what the effects will be with the skull white?
So you're simply worried that a white undercoat will ruin the red.
Use Mechrite red, it's a foundation colour that will lay a flat red down really easily. From there, just go back over them with Blood red or whatever colour you're after, will work a charm.