I have some problems determining the difference between a few skills that all have simiral uses.

Tech Use: Use, Build, Understand and Maintain.

Trade (Armourer): Design, Upgrade and Build.

Trade (Technomat): Maintain and Repair.

Common Lore (Tech): Soothe machine spirits.

Forbidden Lore (Archaeotech): Understanding complex tech

Logic: Understand complex tech.

Security: Bypassing locks wether they contain machinespirits or are normal mechanical.

Security i can see the obvious difference but it still falls under tech use for me. So for me the only actual skills something like a tech priest needs are Tech Use and Trade (armourer). You can add Forbidden Lore (Archaeotech) for special macguffin-tech, Security for BnEs and logic for Ciphers and random cool techpriest stuff. But if you ever get any skill in tech use Trade (technomat) and common lore (Tech) is worthless right?

Got the same problem with Logic and Scholastic lore (ciphers) if you get logic you never need the scholastic lore.

Sorry this got kinda ranty...

Any thoughts?

Well, there are definitely huge overlaps between these skills. I would rule it in a similar way like described for vehicle repairs in the DH2 Core Rule book. There you can use different skills for the same task but with different levels of Difficulty and in addition you have to have some "basic skill set" to be allowed to do a task but if you have additional skills again the level of Difficulty will be lower. I think you could also use this approach for your example of Logic vs. Scholastic Lore (Ciphers). Let's look at a concrete example: If a character only has the Logic skill it will be possible for him/her to decipher an encrypted message but maybe not so easy, say -20 on the Logic test. If the same character only has Scholastic Lore (Ciphers) it will be much easier, say +0 on the skill test. Then again if the same character has both skills he/she may choose which one to use and gets +10 or so. I hope this makes sense.

In addition you could also modify the time needed for a task depending on which skills and how many appropriate a character has.

This is of course not RaW (but the rules on such things are pretty vague). Still, I think when you use the repair rules from the vehicle section as a template you can easily adapt them to fit other use cases as well.

Trade includes a certain amount of "industry knowledge" - Trade (Armourer) would be the appropriate skill for identifying a specific pattern of weapon, for example. Trade (technomat) for a given set of tools, or - given the layout of a workshop or tool station - trying to figure out clues about the owner/operator (occasionally important in an investigative game).

I agree with @eltom13 ; the same test is possible with different skills. But it's also going to be different difficulties with different skills. Tech-use covers a multitude of sins, but for precisely that reason, the test would be easier with a different skill; an arduous tech-use test might be a simple forbidden lore (archeotech) test to operate a teleportatium, or an easy trade (technomat) test to perform a standard overhaul a ground-crawler.

It's the same with 'nested' lores: theoretically Common Lore (Imperium) knows about the Imperium as an entity. So he almost certainly knows that there is a thing called the Ecclesiarchy, and maybe some vague details about it's organisation and belief but he won't much about it specifically unless he also has Common Lore (Ecclesiarchy) and/or Common Lore (Imperial Creed), so the same question may be a harder test.

Not wanting to necro for the **** of it but I'm making a 'field sage' adeptus mechanicus operative assigned to a inquisitor's band and wanted him to be able to create simple things like oils, basic fuels and explosives if need be.

I took scholastic lore: chymistry to reflect this knowledge and of course I started with tech use which I raised to +10. So would I make a chymistry role to figure out if I can make it from available sources and then make a tech use rule to produce it?

Another RPG system I like has a rule that might cover some problems people seem to have with things like Medicae and tech use: the complimentary skill rule. If you have a skill and in a particular situation another skill might compliment it, the complimentary skill lowers the difficulty. So my field sage might be able to create fuel or oil with tech use but the chemistry lore would lower the difficulty. Same for a Medicae trying to create a medicine.

Along these lines I took scholastic lore adeptus mechanicus despite having common lore adeptus mechanicus as I assume he would have a more in depth knowledge of it that any common lore. I assume scholastic lore would help him prove he's genuine and not just someone who knows common knowledge of the AM.

As I have written above (some time ago) compare the vehicle repair rules in the DH2 core rule book. They also define what you call "complimentary skill" (although they use some other term, that I don't remember at the moment) that if you have it lowers the difficulty.

Would it have killed ffg to explain this in a FAQ file?

BTW Eltom13 ,thanks for your answer. I hadn't looked up the vehicle rules yet as I wasn't planning on adding any till later but they were of help to me on this matter.