When I remove solder for reusing copper pipes I get my thick leather gloves over cheaper cotton cloves
Then I heat the pipe until all the solder is liquid and I wipe the pipe off with my gloved hand
I use a propane torch on max when doing this
The gloves get a bit hot but not bad if you wait between pipes
There is a slick of solder left behind but when doing pipes that’s not an issue
On your gas tank I would use the exhaust of a vacuum cleaner going into the hole to vent it for a while first
Then I would heat that solder up until it’s very liquid using Oxy-propane and wipe it all off
You dont have to worry about the traces of lead if you are going to braise the new connection
But if you plan to tig or mig then yes you do need to worry about contamination and sand the final lead off before welding
For this situation I recommend braising it on with the Oxy-propane or oxy-acetalyne for strength.
Just a propane torch might not get you hot enough for this large heat sink tank
Electric will never work on a large surface area like this tank you will loose too much heat and make another cold joint.
-s
