Using 18650s without wrap is dangerous since whole body is actually negative terminal. If you use it without wrap your 18650 could short and potentially caught on fire or explode.
If you salvage 18650s from battery laptop battery you could use this guide to rewrap your batteries since most of them are glued inside and only way to get them out is to damage wrap.

As a safety precaution do this when battery is not fully charged, that way if you accidentally puncture or short it it won't release all of it's energy and catch on fire or explode.

Do this on your own risk and please read carefully, if you don't know what you are doing read more about lithium batteries or ask for help.

Step 1: Tools & Materials

You will need :

Heat gun or hair dryer ( lighter might also work, but i wouldnt recommend it)

Ruler or calipers (be carefull if they are metal, so you wont accidentaly short battery)

Hobby knife or scissors

Wrap (i used PVC, 5m - 2$ on ebay)

18650

Paper rings (if old one is damaged)

Step 2: Cutting Wrap

When flat it is 30 mm wide, inner diameter is few milimeters wider than 18650, so about 18.5mm. It is thin so it should fit in flashlights or e-cigs.

I like to cut them 3mm longer on each side, so about 71mm, maybe even 72mm (72mm in this guide)

Protected 18650 are a bit longer, they are not 65mm, they might be 67-68mm so just add 6mm and it should be fine.

Use ruler to measure 72mm and cut with hobby knife or scissors.

You can also make template, and use it to cut wrap instead of measuring 72mm every time.

Be careful to not damage wrap, if you damage it you would need to cut another one.

Step 3: Remove Old Wrap

When you remove wrap cut down on negative terminal, if you cut on positive you might short battery.

If your wrap is damaged you might peel it off with your fingers.
Try not to damage outer casing, or puncture it.

If it has protection circuit, it will have flat piece of metal that goes to top of battery, dont touch that metal and dont bend it. It is dangerous to touch that metal, if you remove insulation it will short battery.

If your wrap is not damaged you can leave it on, but it may not fit in some devices, or it may get stuck.

Only remove paper ring if it's damaged or if it just falls off.

Dont lose paper rings, they will protect battery to not short positive and negative terminal, if you lose it you can buy another one, or maybe even cut it from cardboard or hard paper.

Step 4: Rewrapping

If you removed paper ring, now it's time to put it back on top of battery and put wrap on it.
You should have 3-4mm on each end, depends on how much you left it (3mm in this case).

You can try this on old battery, or 18650s that dont work, even on 18650 that are working, you wont damage them if you dont heat them too much.

Once you put wrap on battery and align it, slowly apply heat to top of battery,swipe gun or dryer from top to bottom and rotate battery.
This should take few seconds, if you leave it too long it will melt wrap.

Try not to heat battery for long time, if you can hold it its not heated too much.

When you are done, you can put some stickers with original info, or maybe write with marker, its your choice.