Ha, ha - an expert I am not Just a lot of experience.
The texture of primer will show through the BMF. If a primer is necessary, such as at that nasty joint at the engine nacelles of Revell's bombers, then it must be sanded smooth. If you you need to remove a piece of BMF you will need to remove the adhesive before applying new BMF. Use rubbing alcohol to remove Matte Aluminum adhesive and use Simple Green to remove Chrome adhesive. Be careful, the alcohol may soften painted surfaces.
If you need to apply BMF over the edge of a painted surface you will need to sand the paint to a smooth taper.
What are you using to burnish your BMF? The absolute best burnishing tools are paper stumps (burnishing sticks) available at craft stores. They are perfect for smothing and removing that funny pebble texture of BMF. They are soft so you can press really hard to produce a super smooth finish.
I also use a flexible strip of plastic to guide my Xacto knife when trimming BMF aroung curved surfaces. I am using my old lettering guide from High School.
I also use a rivet tool from RB Productions to make rivets. It works great on 1/48 scale models.
Last but not least, I use a piece of 0000 steel wool to enhance the grain of the BMF. Just rub it very lightly (do not press hard) over the foil. A single pass only.
And this makes the tedious task of BMF much easier.