But in fact there are many reasons not to soak, ranging from ineffectiveness to the all too real possibility of irreversible damage. Pens may be made to hold liquid, but it doesn't necessarily follow that they were made to be submerged in it. Pen repair professionals resort to soaking very selectively. In those cases where soaking is done, the pen's materials and construction are positively identified and thoroughly understood beforehand, and the water exposure is held to a minimum in terms of both area and duration.
What can go wrong with soaking? Many pen materials can be harmed by even relatively short exposure to water. Most notable are casein-based plastics, which will swell, distort, and split (they will not, however, actually dissolve). Hard rubber (ebonite) will instantly fade if its surface has had any significant exposure to light over the years. Despite what many say, this fading doesn't require a prolonged soak: even spattered waterdrops will often do it. Celluloid is more water-resistant, but is still much more permeable than newer and harder plastics. Light-colored celluloids can stain if left soaking in inky water, and celluloids with metallic veining or marbling can have their gold or bronze-colored parts turn dull and chalky with water absorption.
When pen people talk about soaking, what is meant isn't always clear. The most problematic sort of soaking is the indiscriminate dumping of the entire pen into water. Caps are prone to accumulate dried ink inside, but it's best to clean them out with moistened cotton swabs and brushes of the sort used for cleaning test tubes. One should not even consider soaking caps with nondetachable metal parts – above all clips, but also trim rings. Introducing moisture is a lot easier than getting it all out again. And once it's there, it will promote corrosion (including on plated and gold filled components, which always have some exposed base metal). For related reasons, it's also a bad idea to immerse a cap with a separate inner cap. Once water gets into the space between the inner cap and outer cap, the tightness of the fit plus capillarity will ensure that it will be there for a very long time. This is of particular concern when the outer cap is made of celluloid, and especially when the celluloid is light-colored or translucent. Staining is one possible consequence, but so is increased visibility of the inner cap. And with certain celluloid caps, notably Wahl-Eversharps, the presence of soft rubber washers makes the introduction of moisture especially hazardous, unleashing the same sort of discoloration more commonly caused by deteriorating ink sacs.
Routine soaking of barrels is also a bad idea. Removing a hardened sac is best done dry, using hooked extracting tools and cylindrical or tubular scrapers. And water inside a barrel can do serious harm to any metal parts inside – harm that may take some time to become apparent. Springs and pressure bars can corrode, while the C-shaped retaining springs typically used to hold levers in place are notoriously prone to rust. Even when the interior of a lever-filler barrel is quite dirty, there's little to be gained by scrubbing it clean, and much at risk. Pens with barrels which directly hold ink, whether eyedropper-fillers or pump-fillers, do need to have their interiors scrubbed out, but this doesn't require soaking, or at least the soaking of anything but the interior.
Some may object that they don't soak the whole pen, only the section joint, standing the pen nib-down so that the water line only just covers the line where section and barrel meet. This minimal approach is certainly far better than full immersion, but still deserves critical examination. To start with, what is soaking of the joint supposed to accomplish?
If the pen is a dropper-filler, the joint will be threaded, and if it is stuck, the likely cause will be dried ink in the threads. Targeted soaking of the section joint will soften the dried ink, allowing the section to be unscrewed. This is one of the few cases where soaking is entirely appropriate, though it must be noted that it is likely that the hard rubber will end up faded where soaked, and will have to be polished to restore its color. To avoid the need for polishing (when the section bears imprints, for example), there are alternatives, such as introducing water into the interior of the barrel with an eyedropper and allowing it to soak into the joint from the inside, putting a drop of naphtha into the joint from the outside, and the application of heat.
If the pen is not a dropper-filler, more than likely it is something other than ink that is preventing removal of the section. If the section screws into the barrel, that something is probably shellac or a rosin-based sealant-adhesive. Water won't have much effect in such cases, but dry heat will (as will certain solvents). If the section is a press-fit into the barrel, the same may apply, though often it is mostly friction holding the section in place.
Sometimes soaking will allow a tight press-fit section to be extracted, the water in the joint acting as a lubricant. What has to be considered, though, is that lubrication of a very tight joint can also have the effect of making it just that much easier to crack the barrel at its threads. Hard rubber doesn't shrink over time, while plastics such as celluloid and cellulose acetate do, so the effects of age often leave the barrel mouth strained right up to its limits. Applying heat allows that stress to be relieved while greatly reducing the material's brittleness. The extreme case of this is red hard rubber, whose brittleness can be so extreme, one should never attempt section removal (or replacement) cold.
Some may protest that certain pens can be safely soaked, and that some components of some pens may have to be soaked. That, however, doesn't address the key points here: that soaking should not be done indiscriminately, nor should it be considered as the initial default action when servicing old fountain pens.