So here's a few things about scarifying a lawn that most people don't mention before they go for it, you have to take into account the condition of the lawn, the time of year, how ready are you to wait for it to recover and what result do you want to achieve by doing it.
The last bit is the most important thing, what do you want to achieve by scarifying your lawn.
I always feel that you scarify the lawn so you can add to it, not take something away which leads me to the 3 ways to scarifying a lawn.
Way 1: Lightly so the lawn can breath.
This requires lifting the scarifier blades so you're verti cutting the grass. Think of this in the same way you mow the lawn, but you're cutting down through the grass. This is the type of scarification we'd do on a lawn in good condition once or twice in the growing season.
Way 2: Extensively because it's time to start a new lawn.
Ok, so the lawns seen better days, its full of moss (which you have had treated), so it's time to get all the moss out and a load of old dead thatch, it's a double pass, and you'd do this if you were reseeding or renovating a lawn.
Keep in mind doing that is very damaging and after care is vital, if you do this and feed and seed a lawn correctly afterwards, and really care for it you'll have a great lawn, but done at the wrong time of year it can take an age to recover.
Make sure you ask about the after care if your planning to do this.
Way 3: Just enough to help the lawn grow.
This is a deeper single pass, just aiming to open up the lawn all the way down to the rootzone. Done well it's a nice job and creates a good opportunity to over seed.
Having scarified lawns over many year, lawns that have been in all sorts of condition, we know there is not one way to scarify a lawn.
Here's today's podcast: