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I always kind of got the impression that anyone with psyker potential needed some kind of outstanding self-discipline (like the Eldar, or soul-bound astropaths,) to avoid daemon infestation. But, well, if you have Orks actually building technology because they genuinely understand it, then they don't need to all be psykers.And the last Ork codex has a blurb that does feature a daemon coming out of a weirdboyz 'ead whilst in the warp. I see their protection as the same as the Sororitas or, perhaps as a closer analogue, the Harlequins. Neither group is otherwise protected, but displays what could be seen as latent psychic abilities (Acts of Faith) or are actually psykers with no outside protection save their god (Harlies).
Though, in that case, it's hard to see how they wouldn't rapidly outstrip the imperium in the tech arms race. Hmm.