That should work .
That is if you have bare steel and do it that way .
If there is already color down , and you just sand it ( not going through the paint ) then spray the DP90 ,
no moisture penetration should get to the metal .

I have see the Horror stories of guys taking their rods to bare steel , then just spraying the DP 90 and running it that way for a year or so , then they just sand it , and lay color , then they start seeing these little tiny bubbles coming through the paint.
Thats rust popping through the paint , as when you sand it down to see whats under it , there are tiny little surface spots starting .

I have also see this at the shop , when we have had a bad hail storm , and the body men have had to sand the roof down to bare steel fixing the dents , and it being hot here in KS . They will lay their sweaty forearms on the raw metal , not thinking , it drys , then they send it to me , and I wil wax & grease remove the panels , as it will not remove the salt content of the sweat , and I will prime over it , prep the panels , and paint it , then about 6 months later the customer will bring the car back because they see these tiny little bubbles showing , and sure enough , when I sand it to the metal , there is rust forming .
So becareful where you lay your hands , or drip sweat , as raw metal can get a hazing of surface rust quick , even with just a minimum of humidity !

SprayTech