I have been priming before dimpling on my RV-7. Here's why: Those dimples will absolutely eat a scotchbrite pad!
My first priming session, I dimpled before priming, and I burned through seveal scotchbrite pads on the back side of the dimples. So, I decided to try it without, and I haven't had any issues with the dimpling, or parts fitting together. Priming prep-work is SO much faster without working around and in all the dimples, and extends your scotchbrite pad life by a good margin.
The only downside, if you are not using a good primer, you have to be pretty careful when you dimple, so you don't scratch up your new primer moving the parts around to dimple. I am using AKZO and this stuff is tough as nails, and very hard to scratch, so it makes working with parts really easy after priming.
The actual process of dimpling hasn't changed for me. I do have to ream an occasional hole every now and then, where the primer has collected in the hole, but its only happened less than a dozen times overall. The dimple die still sets a very nice and clean dimple, and the dimpled parts still fit together nice and tight. AKZO will withstand being dimpled and not flake off as well.
I'll do my whole plane like this from now on
You can check my build log in my signature if you'd like to see photos of it. I have a whole category on priming alone....just did a few parts tonight actually.