Which Loctite?

Have never used purple, have to look that one up for my own education. Blue, it would appear, is the norm, and I use it exclusively for every thing from guard screws, to scope bases and rings. However, before using it, I always wash the screws in alcohol, and clean out any threads with a Q-Tip soaked in alcohol, to remove any foreign material and especially any oil or grease. Never, ever use the RED Loctite unless you want it "permanent". YMMV

ph.iloctite.com/en/red-loctite-threadlocker

I've used red, green, blue, and purple Loctite through the years. I use blue the most frequently since I usually want the ability for easy removal. And I must say that probably 90% of my use is for motorcycle restoration. I strictly use the purple Loctite for very small, fine thread screws or bolts, generally when torque values are listed as in-lbs. Obviously this works great on screws similar to scope mount rings.

I know this doesn't really apply to gunsmithing, but I still use red fairly often on high vibration fasteners like dirt bikes and Harleys. Removing fasteners previously dabbed with red Loctite is no big deal under one major condition-the area must be able to withstand the heat of a torch!

I use blue the most, red for larger things that I can heat to take apart, and green for small screws.

If the screws and mating surfaces are degreased, you can apply green to the assembled parts. Green will wick down into the threads - no disassembly required.