Use liquid cleaners to remove oil/grease from the joint area but. sanding or a stainless wire brush work best to prep the joint be it for soldering or welding. Steel wool can be used but it cannot have any additives that keep the wool from rusting. I have used the 2 inch Scotch Bright pads and a air tool.

Soldering solid wire I wrap the wire together using 19 gauge steel mechanics wire. Sand the solid wire in the area to be joined then sand a length of the steel wire then wrap. The nice thing is that wrapping it with the steel wire will hold the whole thing in place and allow you to adjust everything before soldering. Don't leave it to long even overnight can start the oxidation process affecting the solder job. A few drops of acid flux and solder.

Its not so much the wattage of the soldering iron as the mass of the heated tip. An 80 watt is plenty for even 1/4 wire if the tip is large enough to hold the heat throughout the soldering job. A 200 watt soldering gun is a poor choice if the heat is wicked away once the solder begins to flow and the solder job ends up incomplete and results in a cold joint. First three pictures, first the two pieces of wire to be joined and the steel wire wrap and heavy duty soldering iron. Second the 1/4 inch wire and steel wrapping all sanded and ready to go. Third the joint wrapped ready for solder.

Last three are the float gear for my Piper Pawnee. Little lighter wire all around. You must size your wire to the job and make sure the solder tins the heavy wire not just the wire wrapping. The joint needs solder right through it.