Q. If we already removed the old paint and cleaned a cottage deck before the 3 weeks began, can it be finished, to avoid making it all dirty and having to start all over again?

A. See question 321 in this forum in regards to repairing a basement from a water leakage, where we wrote: “Shulchan Oruch (O.H. 551: 2) rules that from Rosh Chodesh (Av) we lessen the renovation or building of a joyful abode or redecorating, and painting. Mishnah Berurah (ibid. 12) explains that this refers to a residence one does not need now. Mishnah Berurah adds that if a Gentile was hired prior to Rosh Chodesh and he is paid for the complete job and works on his own timetable, he is permitted to work even on Tisha B’av itself. (As far as Mareis Ayn is concerned, see Nitey Gavriel – Ben Hametzorim 28: 7). However it is preferable if he could be compensated with a small sum to stay the repairs for after the nine days. Horav Shlomo Miller’s Shlit”a opinion is that basic repairs that just permit you using the damaged basement area are permitted. However, unessential area expanding and decorative improvements should wait until after the Nine Days.”
In your particular case, Horav Shlomo Millers Shlita opinion is that you may finish the work in need before the beginning of the nine days.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a