I am able to use base parts sometimes, but not always. Started upgrading a torret, but NO option to use base parts to speed up. So, in what circumstance can base parts be used??
Base parts can be used to modify turrets and other buildings, but base parts can't be used for upgrades once your turrets reach level 11. They can also be used to upgrade other buildings to a certain level.
Aside from what Kris said, they cannot be used with new constructions. Only upgrades and refits. And the upgrades are restricted at the current top OP tech level.