BEFORE you do the waxing..., get some
Evaporust. Fill the canteen with it for about an hour..., then pour the solution back into your jug to use again.
Next, rinse with water, followed by a rinse with rubbing alcohol, to help dry up all of the water. Some folks stop at this point and simply deal with a little rust in the canteen.
On the other hand, you can put some beeswax on the stove in a double-boiler to melt. Place the canteen in the oven at the lowest setting, at 170 degrees. You're still going to get a bit of flash-rust, but you want the inside of the canteen dry.
With it dry, and hot, pour in the beeswax (you'll need gloves) and swish the beeswax around and it will coat the inside as it cools. Keep swishing until you hear it stop sloshing around. I've seen it applied by coating the inside of 1/2 of the interior of canteen, then quickly adding melted beeswax to do the other half of the canteen's interior. I've also seen it done by adding the melted wax, and with the cork inserted, the canteen was held sideways, spout parallel to the floor, and quickly rotated side over side by gloved hands to coat the interior. Be careful that you have a stopper that is attached to a stout piece of twine for as the temp cools sometimes the stoppers get tight.
Inspect the interior, and if it's pretty well coated you're done. If it didn't really work, instead of adding additional beeswax, just heat up the canteen, and the wax inside will melt, and you can try again. Be careful of the temp, try not to go past say 220 as you don't know the melting point of the solder in that canteen.
NOW on the outside, you can deal with that rust after you've waxed the inside. Simply put water in the canteen and the cork, and take the used Evaporust, and submerge the canteen in the solution until it just covers the top (but not the spout) and let that sit for like 30 minutes. Remove the canteen, rinse with water and dry. Dump out the internal water, and rinse the insides too. Then let the canteen sit inverted, spout down and open to air dry. Keep the Evaporust solution for another day.
LD