Seal Compression Cap?

I am removing a sink and trying to cap off the copper supply line. I have easy access to where it branches off, and it uses a compression tee there. I think the cleanest thing would be to just undo it's branch from the compression fitting and put what I've seen called a "compression cap" on the threaded connection there.

I understand that a a normal compression fitting uses the ferrule to seal. But, how do these end caps work when there's no pipe underneath them? Do I need to use tape or dope? Do I put a washer under there?

Thanks

cap

They make compression caps that screw onto the tee, but their success rate at not leaking is not very good. Cutting a washer to fit inside may help it seal better.

Thanks, what do you think about putting some pipe dope on there too?

I tried using just the compression caps and they leaked. I cut some rubber discs (used an arch punch) and they sealed quite well.

I do not recommend using pipe sealant on anything but tapered pipe threads.

A great application for a sharkbite end cap!
https://www.cashacme.com/product-category/sharkbite-plumbing-system/

Available at plumbing supply houses and Home Depot.