Wet sanding with 400 grit is the best approach. As far as the guide coat, it will show you the low spots, which may need additional primer or spot putty. You may not want to sand the guide coat off completely, as you may start seeing bare metal that needs to be bumped down.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.