Fiberglass pens aren't that good at removing intact soldermask. You have to scrub really hard and the area cleaned isn't well controlled. However once you have broken up the soldermask with a sharp scraper they are great at removing the residue and getting down to clean copper, or for removing oxidisation and/or corrosion from bare tracks and pads. They also do a reasonably good job cleaning tarnished/oxidised DIP pins and solder tags. A further use if set a bit longer for a less harsh action is as a heat resistant brush for wiping molten solder off pads or as a mild abrasive for dealing with tip tinning failure that isn't solved by using a block of tip tinner cleaner.
You'll also want a selection of grades of Wet&Dry abrasive paper, from about 200 grit on the coarse end down to 1200 grit on the fine end, some pocket sharpening stones (medium and fine grit), some brass wire brushes for cleaning contacts (a pencil one, and a larger but still small rightangle one with a handle). A set of plain needle files and a set of diamond needle files will also see a lot of use.
What gets used on what depends on accessibility, the surface finish desired and the hardness of the material to be removed. e.g. a folded strip of fine grit Wet&Dry paper is useful for dressing burnt open frame relay and contactor contacts in-situ, but afterwards, wet cleaning with alcohol or contact cleaner on a strip of cartridge paper is *ESSENTIAL* to remove any residual grit and debris from the contact surfaces.
A Dremel (or clone) with a wide selection of abrasive accessories is also useful.