Yes - per El Accumulatatorador, get the dirty pad soaking in something soon as you pull it off the machine backing plate..
Also, you need to push the entire pad under the cleaning solution so it can soak it up into the pores..
As for knowing when to change pads, first off, you will be using a clean towel to wipe the face of the pad after you finish working that section..
This will help keep the pad cleaner longer before you have to replace it with a clean one..
You will know when its too loaded up to work any longer, when it just doesnt work any longer on that section of the panel..
And if its single stage paint, it will be full of that color and it will be embedded into the pores..
You need lots of pads to do any single stage paint or any Airplane..
Good luck !
Dan F