I cannot tell you how happy I was to get to this stage of the project! At this point, I have the body panels as straight and flat as I can get them, and now it is time to remove the 220 grit sand scratches from the primer so that the surfacer will have a minimum amount of filling to do.
I started with the roof which has caused me the most grief in trying to get all of the small dings and dents out of it. 1st order of business was to coat the roof with 3M Dry Guide coat, boy I love this stuff, then wet a decent sized area with warm water and Joy soap, which acts as a surfactant to better wet things. I also filled a spray bottle with warm water and Joy as well. I have several 5-6″ soft blocks to which I attached the 400P wet/dry adhesive backed sandpaper to. I used the thicker blocks on the flatter areas and the thinner conformal blocks on the curved area. I spray the paper with the Joy solution to keep the paper wet and clean while using a light pressure to glide the block over the guide coat until no black showed.
After I finished the roof I was very surprised to see that the primer was now shiney. Years ago when I used 400 wet paper it did not leave the primer shiney. I did a quick search on the internet and found out that the ‘P’ grits used a European spec and that they were in fact finer than the old USA standard. As it turns out the 400P grit is more like 600-700 grit in the old rating system. This gave me a bit of concern so I reread the tech sheet for the PPG surfacer that I planned on using. It called for either dry sanding by hand or machine using 400 grit, or wet sanding by hand with 600 grit. I did a further check with a really good body/paint guy on Hotrodding.com who I trust. Mike told me that I would be fine with the 400P paper and then suggested that when I was doing the final clean prior to laying down the surfacer, to wash the panels with warm water and Joy but use a grey Scotchbrite pad and a bit of scouring powder to leave the surface clean and slightly scuffed. Then do a final clean with wax and grease remover just before the final tack down.
So with that I proceeded in the same fashion with the rest of the body panels. Wouldn’t you know it, I had a couple of small high spots on the bottom of both of the front doors! After all of the time I spent on them, arrrgh! I also had a couple of high spots on both of the splash pans that needed to be delt with as well.

Wet Sanding with 400P Grit