A quick question - Techmarine can take 'the flesh is weak' on the first rank, with this he gains the machine trait. Does it mean he is from now on also resistant to all mind influencing psy attacks? (as per the machine trait description)?

Short answer...yes

i wouldn´t use it that way. At least not for the first 3 ranks

How you use it is you's to decide.
But RAW as well as RAI state that the flesh ie weak makes immune to mind influencing psy powers.

Some time ago I sent the question to FFG:

Message from:
> Umbranus

>
> Rule Question:
> When a Techmarine learns "the flesh is weak" he gets the machine
> trait.
> There it states that creatures with the machine trait are immune to
> mind-influencing psychic effects.
>
> Does that immunity apply to a Techpriest with just the flesh is weak
> 1?
>
> Thanks in advance.

Yes.

--
Ross Watson
Senior RPG Developer
Fantasy Flight Games
[email protected]

If you don't like the RAW answer then I'd suggest that each level of Machine gives a cumulative +10 bonus to resisting mind-influencing psychic powers. This is limited to the maximum modifier of +60, but it still works out rather well.

It sounds kind of dangerous to me. Anybody immune to mind influincing psychic powers would probably have enough machinery in him to be vulnerable to machine curse.

What's a machine curse?

Anti-machine psychic power. Also, said bettle brother should watch out for EMP grenades.

Every battle-brother should watch out for grenades which instantly entomb them in 400lb of ceramite and stop their senses from functioning.