How and Why Pad Size Affects Pad Size Affects DA Polishing

[Todd@RUPES]

How Pad Size Affects DA Polishing

DA Polishers, both Random Orbital style, such as the Porter Cable 7424 XP, Griot`s Garage 6 Inch, and Meguiar`s G110v2, as well as Forced Rotation style, like the FLEX 3401 VRG and Makita “Two-Mode” combine the spinning motion of a rotary polisher with a tiny, orbital movement similar to applying wax in tiny circles (only much faster).

For these machines to efficiently correct paint they need to be able to transfer both movements from the machine, through the backing plate, through the pad, to the paint`s surface.

A large number of polishing pads are available for Random Orbital DA polishers. With the appropriate type of backing plate almost any pad type will fit. The question then becomes, “What pad size works best with a Random Orbital Polisher?”

The short answer: A smaller, thinner pad is going to be more powerful and allow for more polishing power. If that is all you needed to know, then feel free to skip the rest of the material. If you are interested in how & why, then keep reading.

The Shock Absorber

For maximum polishing efficiency, the pad has to move across the surface. This creates the friction that is responsible for polishing the paint.

The two motions of a Dual Action Polisher cause the backing plate (and pad) to move in different directions, like a “Scrambler” ride at the county fair. For half of the orbit the pad is moving in the direction of the spinning motion. For the other half it is moving against it. (diagram below) Since the backing plate is hard mounted (rigid) to the machine, it transfers all of this motion to the pad. This is similar how a bumpy road transfers it`s texture to the suspension of an automobile.

Polishing pads, particularly softer foam polishing/finishing pads, have some flex built into them. They must contour to the body panel being polished. As the DA Polisher moves the backing plate, much of the movement of the machine is absorbed by the pad like the shock absorber in a car absorbs the texture of the road. As you drive down a bump road, you are isolated from the road by the suspension and remain fairly stationary. As the polishing pad absorbs the movement of the machine, the pad on the paint can loose much the orbital motion. In a car this is beneficial, when machine polishing it is not.

The polishing action (movement) of the machine is transferred through the backing plate to polishing pad. Some if it is naturally absorbed by flex of the pad. For the most efficient polishing action we want to transfer as much of this action to the paint. Pad size has a direct impact on how much motion is absorbed by the pad.

Height/Thickness

Most foam polishing pads are between 1.5 inches and .875 (7/8) of an inch thick. The taller a pad is the more polishing action will be absorbed . The orbital distance (“stroke” or “throw”) of most DA machines is between 3/16” and 5/16”. To prove this point, imagine we had a pad that was was 10 feet tall. If you grab the top of the 10 feet tall pad (standing on a ladder of course) and move the top of it in tiny 5/16” circles that upper portion of the pad would move, like a bowl of Jello jiggling on itself. However the bottom of the pad (the face) that was contacting the paint wouldn`t move at all. All of the orbital motion would be absorbed easily in the enormous height of the pad.

Taking the above example to the extreme, imagine the pad was less than a .25 of an inch (¼) thick. As you moved the pad in tiny circles, almost all of the motion would transfer directly to the paint. It would be very hard to notice any absorption of the movement, leading to a very direct polishing action.

A thinner pad (1.25” or below) is simply more efficient at transferring the movement, and thus increasing polishing power.



Diameter

A larger diameter pad, because of the increase in surface area, will also absorb more orbital movement, although the effect is not as pronounced as with pad height. With Random Orbital DA Polishers, the pad`s spinning movement is created by centrifugal force. The spinning movement, since it is not directly driven, is slowed (or stalled) by excessive drag on the face of the pad. Using a larger diameter pad focuses the Random Orbital DA Polishers action over a larger area (less focused) which decreases the polishing action.

Using the appropriate sized backing plate, that is the largest size backing plate, that will safely fit the backing plate, will ensure that much of the machines orbital motion is transferred directly to the paint`s surface for maximum polishing power. As diameter (and height) increase, so does the pad`s weight. This increase in weight does rob some efficiency from the machine as it is required to constantly move the pad in a tiny orbit.

Putting It All Together

Selecting pad thickness and diameter is not a perfect science because the requirements of polishing are dynamic. The small guide below will help you pick the right size for your polishing application.

In general, thinner, smaller pads:

• Increase polishing power per level of pad cut
• Reduce the amount of down pressure required to compress the foam
• Cover less surface area requiring polishing smaller sections
• Increase the risk of the backing plate striking the paint

In general, thicker, larger pads:

• Reduce polishing power per level of pad cut
• Require more down pressure to compress the foam
• Cover larger surfaces in less time
• Reduce the risk of the backing plate striking the paint

There is always a trade off when selecting pad size. 3 and 4 inch pads allow the pad to rotate at a greater rate of speed then a 5.5 inch or larger polishing pad, increasing the polishing power. However the smaller diameter means that less area can be covered per section. This is why small diameter pads such as the Lake Country CCS Spot Buff 4 Inch Pads and Meguiars Soft Buff 4 Inch Pads are generally reserved for polishing small areas or when isolated scratches need to be attacked with more power. The thought of polishing an entire vehicle with these tiny pads is daunting, to say the least.

Larger pads allow you to polish more surface area, thus reducing the amount of sections required to polish an entire car. 7 inch and larger pads are a great choice if the vehicle you are working on is in good shape and only requires light polishing. However they can bog Random Orbital DA Polishers down making serious paint correction difficult.

Most users prefer 5.5 inch or 6.5 inch pads when using a Random Orbital DA Polisher. This is excellent size for these tools with a great compromise between section size and efficiency. 6.5 inch pads will cover more surface area, while 5.5 inch pads are will allow for a more powerful polishing action. Popular 6.5 inch pads include Lake Country CCS 6.5 Inch Pads and Lake Country Hydro-Tech 6.5 Inch Pads. With 5.5 inch pads you have your choice of standard height Lake Country CCS 5.5 Inch Pads or low-profile Lake Country Flat 5.5 Inch Pads or Lake Country Hydro-Tech 5.5 Inch Pads.

[Bunky]

I wonder if there is some measure of "loss" due to height. It is almost like winding the pad but at some point it will follow otherwise would sheer apart until there is some direction change.

[Nth Degree]

I wonder if there is some measure of "loss" due to height. It is almost like winding the pad but at some point it will follow otherwise would sheer apart until there is some direction change.

It would depend on pressure and the rigidness of the pad. Though I`m sure if some physicist here was ambitious they could come up with an equation but then we would have to know: Rigidity of pad, thickness of pad, pressure, contact surface area, rotational speed, offset of orbit of DA, distance from contact surface to center of mass of polisher, inertia (mass) of polisher minus counterweight, Rigidity of the arms of the operator, coefficient of friction of the pad and polish to the paint (probably a variable to compensate for ambient temp and humidity) all with a derivative for the increasing temp due to friction over the working time. Perhaps I`m over thinking this. Sorry. I have been out of commission and quarantined to the couch for a few days with Strep throat. Extremely bored.

This concept is important for understanding the importance of always having the pad in contact with the paint before starting it and not over pressuring the pad as both can easily cause paint ripples due to the "winding" effect you mentioned.

[Bunky]

This concept is important for understanding the importance of always having the pad in contact with the paint before starting it and not over pressuring the pad as both can easily cause paint ripples due to the "winding" effect you mentioned.

I am sure the exact calculation could take in all sorts of factors. I was just thinking there may be gross measurement like 10%, 25%, etc in the same way a flat pad has been compared to CCS in contact area under similar conditions like same pad in 7/8-inch vs a thicker one of same time.

I recall there was negative marketing against thin pads a while back. I think they were called "oem" pads. Danase, out of business now, sold thinner Lake Country 5-in (not 5.25-in pads). I still have a few left.

[Todd@RUPES]

I wonder if there is some measure of "loss" due to height. It is almost like winding the pad but at some point it will follow otherwise would sheer apart until there is some direction change.

A winding motion would occur due to the rotational movement (spinning) of the pad, almost like taking a rubber band, extension cord, shoe string, and twisting the ends in opposite directions until the compression was create enough that it could not longer be compressed.

The tiny orbital motion (5/16" or 3/16") is simply absorbed into the pad. This loss in energy (covered to heat by the flex/compression/stretching of the foam) is what leads to the heating of the pad and backing plate on a DA. That heat is energy lost, and while it makes polishing safer, it does reduce polishing potential.

It is one of the reasons that Meguiar`s DA Microfiber Cutting Discs have such a thin, firm foam interface. It allows for a very high percentage of polishing motion to be transferred directly to the paint`s surface, and thus a higher level of cut.