How do you cut out the leak and reconnect the sections in the middle of the fin section

Fintube is not a plumbing term used in the USA, can you tell us better what it is that you are working on?

Difficult job at times since fins are in the way and hard to move. What is the base metal of the tube? Is it pinholed or cracked? If copper, cut it out and sweat a new piece of pipe. If galvanized tubing, you may have to try and braze in a piece of tubing.

YUP! If the slantfin baseboard tubing is copper you'll want to take a sawzall with a metal blade and make 2 cuts about 2 feet apart. If the thin fintube piping dents that is OK, just use a mini. Tubing cutter to cut back another couple inches once the piece is removed.

Then you'll take a pair of aviation snips and cut away say 5 fins on each end of the remaining copper tubing.

Purchase two 3/4" copper SLIP COUPLINGS and then clean/flux the pipes ends and fittings and slide the couplings over the new piece of pipe to be inserted into the space. Once the pipe is in place you slide the slip couplings in place and sweat everything up and you ahould be all set to go.

Note here that you can lose the depth of slip couplings so make a very small mark on the pipes where you want the slip couplings to stop so you are sure the full depth of each joint is correct, OK?

If this is other than copper fintube then post back on the exact tubing material.

Let us know if you have questions...

Mark