What can I do with stripped Power Armour Frames?

I don't believe there's a way to sell your frames. I certainly haven't encountered any, and the fact that the frame doesn't show up in your inventory makes me pretty confident that I won't find any.

You can get out of power armor underwater, so you can just walk the frames into a river if you want to get rid of them. Alternatively just leave them somewhere. Fast traveling does not drain fusion cores. They'll eventually disappear if you leave them outside of a settlement.

If someone puts them on and you kill them, the frame is not lootable from the corpse, so I guess that would get rid of them.

If you decide not to get rid of them, they aren't completely useless. You can save one for any human or synth companion you take with you.

Settlers will get into a vacant frame if they're being attacked and it has a core, so you could leave equipped frames at your settlements to help your settlers with defense. It's plausible you could order specific settlers to enter them from the workshop menu, but comments indicate that this has not worked out in tests.

You could also outfit different ones for different scenarios, but that's probably not a great use of your time, as it's easy enough to just swap the parts on the frame you normally use.

Build up a large collection of different Power Armors? Think of them like cars. Having 1969 GTO doesn't mean you have no use for a 1962 Corvette (or whatever).

Why not try to have a full suit of Power Armor in every style and mod combination? (Or maybe just the loadouts/mods you find useful.) Or different colors? (And so on.) Hell, why not build a Power Armor "show room"/gallery at one of your settlements so you can admire your awesome collection of military hardware?

Seems better than scrapping or selling the old pieces and trying to make the frames disappear.

The simplest thing you can do with Power Armor Frames is to add a Fusion Core to them, and fast travel to an area. Fast travel does not drain Fusion Cores, which allows for a few things.

If you are encumbered, Power Armor adds Strength which in turn grants a higher carry limit. This can be enough to take you out of encumbered state, allowing travel to an area where you can dump all the looted items.

Placing a Power Armor Frame in each of your settlements that have Power Armor Stations, means you can remove a Core from one set of Armor, and travel to the next area with an armor waiting, and power that armor up with the same core. having specialized armors in each area might be a nice idea for some people.

Power Armor resists radiation by a high amount, making underwater travel safer. While not completely safe, it's a good option to have a specialized set just for underwater exploration.

Having Power Armors in each area means you can use the station to craft. without an armor nearby, you do not get the option to craft, even if you have power armor parts on you or in storage.

There's not much you can do with multiple frames, but even a coreless frame makes for a nice stand for the armor. Having a whole line up of robot suits just as decoration and collection purposes is pretty cool.

Edit: As pointed out earlier, settlers might be able enter frames to defend the settlement.

I just built a big shack at sanctuary, put them in a 5x5 array, and gave them all one fusion core, and when ever I find a piece of power armor I just put it on a frame, and so when anyone attacks they have 20 power armor wielding settlers fighting back

If you're playing on PC and aren't afraid of modding, Yukichigai of the Nexus community has created this mod: Move Store and Assign Settlers to Power Armor, which gives greater control over the frames themselves. It allows you to:

• Move or Store Power Armour Frames in the Workshop
• Assign settlers to Power Armour Frames

At the very least, 'storing' them in the workshop gets them out of the way, and that way they're still around later if you ever want to pick up a human companion.

If you pacify a enemy human, command him/her to get in the frame, and then kill them, it should disappear eventually with the body

You can deck out most of your companions in them.

I do this for missions I think will be difficult - I know they can't die but it definitely helps when 5 Deathclaws surround you.