Definatly sand first, then Primer, then top coat.
Krylon should be just fine.
Suggestion though, if you are using chrome spray paint apply several thin coats and allow to dry. If you build up too many coats too fast, the under layers won't cure properly.
You don't want to pick up your nicely painted bucket only to leave finger prints in it. Plus, everytime something touches it, it will leave a mark.
So be patient. Chrome paint needs alot of cure time.
I bought some Chrome duplicolor brand at Wal-Mart and it seems to cure alot faster than Krylon.
Just my experience.
Hope this helps.
Thanks,
Robert E.
Also, if your using the Chrome paint, make sure the helmet is perfectly smooth!
Any imperfections will show.