I use a long handled Kent scraper,its going to be a chore whenever you scrape coraline growth,the scraper makes it a lot easier when faced with holding a razor blade.They are also quite inexpensive and the blade is replaceable if you bend it. I would try and do a little bit at a time working for ten minutes each session,you will have it good as new in no time. Count your blessings, a lot worse things could happen while being gone for 3 weeks I would be THRILLED! if all I had was some coraline to scrape.