Since the SilverLoad is sheathed in UV protected vinyl shrink-wrap, it might not be a great idea to paint it. After the weather and moisture has a chance to work its way thru minute cracks in that paint, it may separate from the vinyl. Then it will look blotchy and nasty after it flakes off. As Beetle sad, metal compounds in the paint might affect it function too. Hate to see how it would work if sprayed with aluminum paint - lol! The SilverLoad is a top-load type antenna - BTW. I would just clean it with soap and water and run it as it is - IMO.
Now, if you were talking about an A99, Imax 2000, or other fiberglass-type antenna; then it is decent plan to use varnish. These antennas begin to shed fiberglass after being weather-beaten after a few years and actually benefit from getting varnish on them. That varnish will act as a binder to keep the fiberglass together for years to come.