Thats a great question. I wondered about that when Jason said it. Where is Todd Helme to explain in his genius type of way?
Here we go… lol…
When we are talking about finish quality, we could likely sum it up simply as what leaves a more level surface. It is important to understand that there is no such thing as a perfectly polished finish… polishing always imparts a scratch pattern on the surface - in order to remove material we have to use abrasives that abrade the surface.
Most polishers fall between 6mm and 21mm’ of stroke/throw/orbit length. What does this mean? The stroke or throw is the diameter from one point of the orbit to the opposing spot. Take a bicycle pedal. If we measure the distance between the pedal when it is at the very bottom of its orbit and the top of its orbit, we would have the stroke length.
In order to examine the effect that orbit length has on the finish we will take each to the extreme and then work to the common lengths. First, imagine you are holding a foam wax pad on the surface of a car. Now imagine you are rubbing in circles similar to the motion Mr. Mygai made famous in “Karate Kid.” Loaded on your pad are tiny abrasives equally spaced 3 mm apart.
If you move in very tiny circles (orbits), perhaps the diameter of the top of a pencil eraser (about 2 mm) what will happen? These tiny circles will begin to burrow into the spots they are repeated rubbed over. The small circles create a “tight” turn or circumference as they are only covering slightly more than 6 mm (pi x 2 mm) for every 360 degrees of turn. This is similar to digging your heel in when you pivot. The combination of focusing your energy into a small area, making a sharp turn, and burrowing in means we will aggressively remove material from isolated spots. The end result is a very uneven surface - hardly the best from a gloss perspective.
Now let’s take the orbit size to the next extreme, and make meter (yard) long orbits. The end result is equally unimpressive for different reasons. The extremely large throw still prevents cross-hatching of the abrasives (spaced 3mm apart) and creates a visible scratch pattern because the marks are linear. Also the area in the center of the orbit size will not even be touched, resulting in a ring the diameter of the pad that has been effected…
So we know that too large of an orbit is ineffective and too small creates an uneven surface. Somewhere in the middle between the extremes is the ideal result we are looking for. So what orbit length creates the best finish?
There are a large number of variables at play. How tightly are the abrasives packed onto the pad, what type of pad is being used, how is the paint reacting to the polishing process, how much speed is being used, etc..
We end up with a dynamic bell curve. The truth is that all of the common orbit lengths tend to work well for finishing quality (larger than 6 mm). However moving larger than the minimum length tends to result in a gentler turn (remember the digging the heel in analogy) as well as removing more material (paint residue and loaded abrasives) from the surface (preventing burrowing).
On most paints, you will not be able notice a difference in gloss when using the different common orbit lengths. However, on difficult (haze-prone) paints you may be able to see a noticeable different - all variables remaining the same. On a difficult repainted G37, if found that with my combination (RUPES Yellow Pad / Keramik Gloss Polish) the 15 and 21 mm BigFoot Polishers produced a noticeably better finish than other tools.
) diff between the 15 and 21? Yeah, I know I'm splitting hairs, but hey...it's Autopia.