What they are speaking of is a Metal conditioner PPG makes ( I have forgotten the Product number ) Its not really required if your metal is clean and wiped down with the DX-330 or DX-440 , then spray the DPLF .
I have been doing this for years with no ill effects, DPLF on bare clean metal bites hard and makes for the best Paint foundation there is IMO .

Most paint gun manufacturers make a special air cap regulator to let you know how much PSI is needed at the air inlet to know your cap pressure .
Using your gun and mixed DPLF start out with 12 -20 PSI on the gauge , or did your gauge come with a color coded needle gauge , if it did stay within the green color and all will be fine .

SprayTech