I assume you are talking about Anti-foul?? After a sand, you need do nothing else but apply the new. Scrapping would have been safer than
sanding, but the jobs done so never mind.
If you are talking about putting an underwater primer/undercoat on to a clean sanded
hull, then yes you can give a wash with a de-waxer or solvent. Altex have a pre-painting cleaner. Most
Paint companies and suppliers will have.