Remove paint which includes primer to the bare metal...
Wipe the metal with an aluminum prep..
Then install the antenna...

Now, some details where the devil lives of how "I" do it...
You can put a tape dam outlining the area of the base of the
antenna... Make the area of the dam just slightly smaller than the
base of the antenna so there is a thin rim of paint projecting under
the base - 1/4" will work... Thin line striping tape from the auto
body store works like gangbusters... Then tape a plastic bib around
that to protect the sorrounding paint... Then remove paint and primer
inside the dam with a paint stripper... Do not sand or abrade the
exposed metal because you will scratch through the cladding (the thin
outer surface of pure aluminum that protects the aluminum alloy from
oxygen and water)... Then put a thin smear of NOALOX on the exposed
metal and install your antenna... It also will not hurt to put a thin
layer of non hardening gasket on the paint rim to seal against water
intrusion - Permatex FORM-A-GASKET #2 has always worked for me... Work
carefully and be neat... Once the antenna is installed than use a rag
with a bit of mineral spirits to clean up the Permatex that squeezes
out...

Now on the inside of the skin where the clamp plate squeezes the
metal, also take that area back to bare metal... No need to leave a
rim of primer here (dry interior)... Just a dab of NOALOX and you are
good to go...

denny