Tips for air-sealing an attic that already has blown-in insulation

I did basically the same thing in my 1960s ranch, kept both layers of blown-in in place and worked around them. I used a leaf rake too but alot of the deep spots I just scooped by hand with a dust pan into buckets, then dumped it back once the area was sealed. Took forever but I didn't trust the rake around the wiring. The thing that surprised me most was the top plates of the walls, exactly like you said. I found a whole run along one bedroom wall that you could feel air moving through with your hand on a windy day. Also had a plumber's chase behind the bathroom that was basically an open hole to the basement. Filled that with a big blob of fire block foam and the upstairs bathroom stopped feeling cold overnight.

One thing I'd add for anyone starting: take photos of every spot you mark before you rake the insulation back over. My chalk marks mostly vanished under the loose fill and I ended up hunting for a couple of patched holes all over again on the second pass. Mine took three weekends instead of two, mostly becasue I kept finding more dirty patches once I started looking. The drafty bedroom is fine now and the furnace runs noticeably less in January.

This is helpful, I've been putting off going up into my attic because I didn't want to make a mess of the fluffy layer for nothing. The part about the gray patches being a clue is clever, I never would have thought dirty looking spots meant air was sneaking through there. One thing I'm still fuzzy on though, how do you tell the old loose stuff apart from the newer layer once you rake back to it? My house came with who knows how many decades of stuff piled up there and I'd hate to scoop out something I shouldn't.

Been through this myself and one thing I'd flag is don't bury the soffit vents while you're raking around. I almost covered mine with the stuff I shoveled back over, then realized the eaves need to keep breathing. Tucked a piece of cardboard or a baffle in front of each one before I pushed insulation back so they stayed clear. Also, do this job in the fall if you can. I started mine in July and the attic hit temps I won't repeat. Cracked a gable vent open for cross breeze and it made a huge difference in how long I coud stay up there per trip.