Also a chisel tip so you have a decent size area of the tip in contact. Cylindrical tips that I see on many soldering rigs make very poor contact. Think about it.

My soldering wire specs 

No-Clean Water Washable

2.2% flux core

0,015" (0.38mm)

Sn63/Pb37

The secret is liquid flux, it transfers the heat from the tip to the structure very fast.

I am using 700F but I recommend 650F until you have enough experience.

My unit is a HAKKO FX-951. It has auto shutoff (programmable) 

I NEVER use anything more abrasive on the tip than the standard brassie included with most irons. I wipe the tip on the brassie when I pick it up and when I lay it back down in it's cradle.

I totally forgot that I had a problem similar to yours a few months ago.  I was trying out some short female headers on a board to hold a Pico.  Two rows of twenty headers and I screwed it up with so much solder that the male pins wouldn't fit.  Ugh.  Long story short I eventually was able to remove the headers but had a problem like yours with lots of the through holes filled with solder.  In this case I tried using solder braid and flux but had more success with a solder sucker.  Something like this one: https://www.adafruit.com/product/148    I was using leaded solder running my iron at 640F/340C and used the largest iron tip that ships with the Weller WE1010A soldering station.

Tom

   Sorry Ron, but I think you might have mixed Fahrenheit and Celsius ... I might push an iron to 400 degC, but not 600+!

And @scsiraidguru, a trick I used on a board, was to add some low temperature solder, before trying to remove it with flux and braid. e.g. tin/bismuth which melts at around 138 degC. I had it as solder paste in a syringe, but it may also be available in coil.

However, removing solder from small pin holes in large power planes is never easy, and damaging the board is always a risk.

Good luck, Dave

You'll probably need to pick up some solder braid and flux to pull out that solder.  I've learned alot from My Solderfix on You Tube.  Check him out at https://www.youtube.com/@mrsolderfix3996

Tom

I bought a new Arduino Giga R1 and want to use the Off, Ground and RTC pins that are soldered shut.  I have a decent solder iron and set it to 350C.  It wouldn't heat them up enough to push a header through.