Hi Jospeh,
Given there is always something new to learn, I wonder if you might be able to do a live session that includes information on using the Brushes effectively.
It has taken me awhile to get to want to use brushes to modify local adjustments and at last it seems to dawn on me what is possible and what is going on.

I ask all this as I been working on some images, think subject, trees in background, out of focus light- white- areas in the background of the light through the trees.
Wanting to make the background darker, I decided to use the Polariser/Multiply brush, and started at it. What threw me was that the white areas began to take on a dark line on the inside of the area toward the edges. Even reducing the strength of the brush didn't help.
So, removed the changes and tried again with the Burn brush. Pretty much the same sort of result.
Out to Photoshop, fix the images, think AP3, is not all that helpful.
But: Rethinking it over night, it came to me that there might be something I was missing.
Back to AP3, open brush, and noticed that the “Detect Edges” is checked.
Uncheck, try again and this time the white areas get reduced along with the green tree/brown foliage.
With Detect edges, it does and so doesn't do any work on the majority of the light areas. But because of how it is set up, it does add some darker edging, which I wonder is dependent on what value I put on Feather?

Strike one up for AP3, and duh for operator.

Be interested in other brush tips.
DJ