necrosis on older leaves

Honestly, if the problem only exists on the lower leaves (and it looks like they aren't even true fan leaves), I would personally whack them off and forget them... 😊 No matter what you do, you're not going to make them better and you just might be doing damage to the upper leaves...

I've had similar damage on a few leaves before. Mine was due to diluted neem oil sprayed on leaves. It was still a little too strong for my more delicate leaf strains. Many liquids left on leaves during lights on if not evaporated off moderately quickly can cause concentrated spots, creating accelerated light burn in those areas. Unsure if this could be the solution but I hope you find it!

Das sind vermutlich mechanische Schäden, da die Blätter ansonsten intakt erscheinen. Weiter beobachten, ist so aber erstmal kein Grund zur Sorge. Viel Glück

check pH... too little info. looks like a boron def, but if you are in soil or using a proper hydro nute line for soilless, it is highly unlikely that you have a problem with trace elements, like boron. so, without any info, probability dictates pH issue or lockout. other possbility easy to eliminate. i've seen spots like that develop from light burn. are your lights too close? if unsure, judge by internode length on plant.. if they are too short, go ahead and raise light 2-4" and see if it stops. Those spots will never heal, but if it stops, you know that was the cause.