Yes, that's actually a good idea if your not planning on applying the surfacer right away. It only takes 8 hours for flash rust to start on bare steel, so the best option is to put an etch primer to protect it.
Just scuff with 320 grit paper or a red scuff pad, and reapply in case you have any thin areas. Don't forget to wait the 30 minutes before applying the surfacer.
John
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