This has veered a bit off topic...but anyhow, it seems experiences vary with regard to all this stuff so you can only put so much stock in the experiences of others. Yeah, that's a disclaimer
I too find the PC ineffective with larger pads. For serious correction, especially on hard clear, I use 4" pads and they work much better for me. For more gentle work I use 5-6.5" ones. I reserve larger pads for the rotary.
I've used both counterweights with all sizes of pads from 4" to 7.5" (and I even played around with some larger Meg's pads). In *my* experience the larger counterweight results in less vibration, especially when mated with light backing plates; lightest combo = least vibration. Many years of use with the larger counterweight has not damaged either of my PCs. FWIW, using my older one with the smaller counterweight (and a 5" backing plate and the old smaller-than-today's pads), the PC required rebuilding after maybe three years; I have no idea whether the counterweight had anything to do with this or not but I'd guess not. I never used the larger counterweight until I got my second PC, which came with the 6" weight. I noticed less vibration so I equipped my other PC with that weight too. It's been over a decade since then and both PCs are still working fine.
As
Spoiledman tried to explain, the PC is designed to work with a backing plate that has *one sheet of sandpaper* stuck to it. A foam pad, let alone one saturated with product, weighs more than a single sheet of sandpaper- if you don't believe me get some and see for yourself. So we're *really* using the PC in a way for which it *was not originally designed*, we're putting more weight and, more importantly, more surface area/friction into the mix. Also, note that when sanding you don't apply pressure, you let the sandpaper do the work, but most people apply some pressure when they polish vehicles with the PC. All these factors combine to make for one mighty outside-the-box situation, relative to the original *intended* application. It's not like PC redesigned their sander when they realized that people were using it to polish cars
