The new PC 7424XP is a great tool and should tackle most of what you come up against on you Mustang. Since you're new to machine polishing, check out this thread as it's pretty detailed in all aspects of using this tool for correction work.
Tips and Techniques for using the PC 7424XP Dual Action Polisher to remove Below Surface Defects
Dana likes to do an IPA wipe down after each step when he details cars and that's his method and it works for him and you're welcome to follow his advice as there is no right or wrong way when it comes to buffing out a car when it comes to a step like this.
My personal preference is to just move forward in the process, that is after using your first step product, in this case the M105, move on to your second step process the M205, no IPA wipe down in-between.
Most chemists formulate their product to have a chemical synergistic compatibility, That is the chemist best knows how to make all the follow-up products in a system because they formulated the initial use or first step products in the system.
I'm not sure I've ever read any car wax manufacture recommend to chemically strip paint after using their products before going on to the next step.
But just to make sure everyone's clear, this is a
personal preference and no one will deny that Dana aka Asphalt Rocket is not good at what he does, so feel free to try his method but I want to point out in this thread for everyone that will read it into the future, that it's not recommended by the manufacture of the M105 and M205 to chemically strip the paint
and it's not necessary if you do a Test Spot first and insure that you're getting the results you''re looking for in your Test Spot.
Then the idea being if you're getting great results in your
Test Spot, (which you can do an IPA wipe down to insure you're getting an accurate reading of your results), then there's no reason to wipe the entire car down if you're simply duplicating what you did in your test spot to the rest of the paint on the car.
Here's a thread that talks about chemically stripping the finish with lots of pictures that show why it's not necessary if you're using good technique to start with.
1-Step versus 3-Step Process by Hand
Test Spot
Here's a thread with a video that explains how and why to do a Test Spot before attempting to buff out your entire Mustang. This video uses a Flex 3401 but the principals apply no matter which tool you're using and even if you're working by hand.
How to do a Test Spot using the Flex 3401
The above is a portion taken from this write-up
1957 Chevrolet Belair Extreme Makeover - Flex 3401 & Wolfgang Smackdown!
White paint can be more difficult to see before and after results because of the nature of light colored cars, sometimes it helps to look at the surface from an angle.
Using a single line of tape on a horizontal surface and then only buffing on one side of the tape will make seeing before and after results easier so you can make sure you're getting the results you want and to give yourself the confidence you need to move forward.
Even though white paint is more difficult to see before and after results, you can see them and if you don't have good sunlight available then another option is the Brinkman Swirl Finder Light.
http://www.autogeekonline.net/forum/new-car-care-products/21256-brinkmann-swirl-finder-light.html
How to use the Brinkman Swirl Finder Light
Using the Brinkman Swirl Finder Light is pretty straightforward but here's a few tips to get the best results when exposing swirls.
You want to hold the Brinkman Swirl Finder Light about 12" to 16" away from the surface you're inspecting and hold it at an angle so that the light bounces back to your eyes.
Correct distance to hold the light from the finish
Swirls in the clear coat finish surrounding the dual Xenon light bulbs
Correct hand position when inspecting for swirls - Comfortable, ergonomic grip
Incorrect position - Puts stress on your wrist
