If you are like me, you buy used pens when you get the chance.  Recently I purchased some used Koh-I-Noor Rapidograph pens.  They were sizes 3 x 0 and 1, two sizes I use often when working with multiple colors of ink.

Below are some photos that show what to watch out for when purchasing used technical pens. In the first photo below, you see three pens and an extra nib that I purchased.

This photo below shows a "typical" used pen.  There is old ink in the ink reservoir, and the nib is stuck in the nib collar.

 Here is another pen below showing typical use.  The cap has dried ink, and there is old ink in the nib weight.

Below is a photo of the extra nib that came with the pens, and it too needed a cleaning.

 The photos below show a nib in a nib collar, and then the bottom area of the nib collar.  The bottom area has to have that rectangle opening clear (sometimes they are a different shape) or the nib won't be able to draw ink.

 The nib shown below is really stuck.  This pen had been used and then left to have all the ink dry up in it.  When they are this stuck, there is no sense in forcing it.  The nib was put in an ultrasonic cleaner for four cycles before it started to free up.

In the photos above, you can see the different pens as I took them apart to check their condition.  That is the first step.  The next step is to put each one in an ultrasonic cleaner, unassembled, until there is no ink residue left on each part.  You could also soak them in Koh-I-Noor Cleaning Concentrate or even water, but an ultrasonic cleaner is faster.  When taking the nib out of the nib collar, be sure to use the correct nib key.  Vintage Koh-I-Noor pens came with a gold-colored nib key, and although it looks similar to the more modern black key, it is shaped differently.

Below is a photo of the ultrasonic cleaner I use.  For many years I had a Koh-I-Noor Ultrasonic Cleaner, but when they became hard to find, I purchased one made for cleaning jewelry.

Ultrasonic Cleaners will run a little hot if used continually to clean several pens.  I leave mine open every few cycles to allow the fluid in the tank to cool off.

After cleaning each pen, I let the different parts dry on a paper towel unless I am going to use the pen right away.  When fully dry, I re-assemble the pen.  The process of taking a pen completely apart and fully cleaning it should be done whenever a pen isn't in use.

One note on vintage Koh-I-Noor pens.  I had a size 0 vintage pen that needed a new nib.  When I started to insert a new nib, it didn't screw in easily.  I thought it just needed to be screwed in a little hard, until I had a second thought and took it out again.  The original nib's weight collar had been in the nib area of the barrel, and prevented the new nib from being inserted easily.  The resistance wasn't that much, but I probably would have ruined the pen if I had forced it.  Try to look inside the barrel of a vintage pen before inserting a replacement nib.

Below is my current work space.  The pens on the left are in use, each with a different color.  The pens on the right are empty and clean and ready to be used.