Intriguing thread, the part about saturation at the center of the pad. Some thoughts (on the fly, so they may not lead to the correct conclusion).
At the dead-center of the pad, what is occurring during the polishing cycle?
To paint a clear picture for the first thought on the subject, let's exchange the pad, machine, and painted panel for a pencil and a sheet of sandpaper. If we were to place the sharpened tip of a pencil against a piece of sandpaper by grasping the stalk of the pencil between the palms of our hands, we could rapidly twist the pencil back and forth by repeatedly rolling the stalk between our palms. If the pencil tip was not allowed to move from its x-y position, we would eventually see lead grindings accumulate around the general vicinity of the lead.
The lack of any substantial lateral motion would result in an accumulation of the lead debris.
Machine stroke (or a lack thereof) can have a a dramatic effect on pad saturation at its center point.
Anyone that's ever used a ZOFRO polishing machine (Zero Offset Forced Rotation Orbital) is well aware of its propensity to accumulate debris and moisture at the center of the pad.
"What's that, you say...you don't know what a ZOFRO is? Oh goodness, forgive me! Being such a big-time proponent of orbital-action machines has caused me to assume that you think like me. A ZOFRO is also referred to as a ROTARY polisher..."
Back on track! Unless the rotary polisher is scuttled along at a rapid clip via the user's arm movement, the center of the pad will eventually load with and moisture and debris.
Turning the focus back onto random orbital machines: if we chose to minimize or eliminate backing plate rotation by utilizing a super-short stroke, the same sort of dynamic would occur. Taking things to extremes, if we were to use a machine featuring a very small stroke (.1mm orbit), and the machine were run at very low speed (1 RPM), and we held the machine in place (or moved it along very slowly), there would likely be little to no generation of backing plate rotation.
Assuming for a moment that there would be zero plate rotation, we would essentially have the makings of a super-short stroke, orbital-action polisher. Barring any substantial user-applied movement of the machine, the accumulation of polishing debris would likely occur wherever we happened to place buffing liquid onto the pad. If we avoid placing our buffing liquid at the center of the pad, then saturation of the center would not occur.
If this seems sensible (and the premise is accurate), then we begin to realize that there is no magic drawing power towards the center of the pad, but rather a lack of motion to assist the movement of polishing debris (abrasives, paint residue, and buffing fluids) away from the center of the pad.
Even with an abundance of backing plate rotation (such as 10-12 rotations per second, equalling 600-720 RPM), the center of the pad is still rotating too slowly to cause the elimination of polishing debris via reactionary centrifugal "force". In regards to any rotary-action or random orbital polisher, the center of the pad will always be the area of the pad that is traveling at the slowest velocity. It's therefore reasonable to conclude that if backing plate rotation promotes the removal of polishing debris and liquid from the pad, then compared to the rest of the pad... the migration of debris and liquid away from the center of the pad is less likely to occur.
Push, force, and roll debris off of the pad.
Anyone that has used a large stroke random orbital polisher has probably noticed that per application of buffing liquid (when using a foam pad), the time length of the buffing cycle can be extended (compared to a machine featuring a shorter stroke). In large part, this has to do with the extended motion, as the entire buffing pad is moving down a longer rotational path. Since the entire pad is traveling more distance per orbit, it is moving at a higher velocity. The added speed aids in increasing the force placed against stuck-on debris, so it is oftentimes more likely to be pushed or rolled off of the face of the pad.
Imagine using a random orbital polisher that featured an orbit diameter of one mile! Pretty silly, but by going to extremes, one could easily imagine that as the backing plate whizzed along such a gigantic orbital path, the pad would appear to be traveling in a straight line. With such a large, swooping motion and all that speed, it would become very difficult for debris and fluid to migrate toward (or accumulate at) the center of the pad.
Twisting + compression + absorption = saturation.
It's likely that a pad's center is the portion that remains in contact with the polishing surface the most. Additionally, it's likely that the center of the pad has more downward pressure and constant force placed upon it (compared to any other equivalent-sized portion of the pad). After all, when polishing panels that are flat, all portions of the pad are in contact with the panel. However, when polishing is taking place on convex or rounded panels, it's likely that the center of the pad is spending more time riding upon the crowned areas of the convex panel shapes.
With constant twisting and compression of the foam comes a capillary or wicking effect that causes moisture (and debris floating in the liquid) to flow into the pad. Over time, the center of the pad becomes saturated, (and any instilled abrasives can create a damming effect that assists in locking debris and moisture in place.
Interesting note: guys that utilize a polishing style in which they apply a bit of tilt to the machine tend to see ring-shaped wear pattern form directly below the edge of the backing plate (because the hard backing compresses the pad's foam, forcing it into the paint). If there happens to be any debris-laden or moisture-saturated pad sections, they are usually along either side of the ridge.
And that's the short of it.
