First, proper use of thread locker will use only a very small dab on the threads and will not effect your servo arm. We do it all the time with servo arms and ball studs on helis.
Q: Will Loctite products attack thermoplastics?
A: Loctite anaerobic products can attack some thermoplastics. Generally, they will not attack thermoset plastics. A cyanoacrylate can usually avoid stress cracking or attacking of thermoplastics. Cure is so fast that it generally will exhibit no serious stress cracking.
Q: Why does Loctite only fill half of their anaerobic 50 ml and 250 ml bottles?
A: Actually, there is exactly 50 ml and 250-ml adhesive in each package. We fill bottles halfway because air is needed in the package to prevent our anaerobics from solidifying. Our 50-ml and 250 ml bottles also allow our anaerobics to breathe by allowing air to permeate through the bottles.