According to Practical Sailor Magazine, March 2013 issue, this topic was discussed. It is generally regarded that after 72 hours of exposure to air that hard bottom paints will oxidize and lose antifouling properties but test are showing otherwise. Of course you should always check with the manufacturer of your paint choice below are the guidelines to reactivate hard paint:
- Launching of newly painted boats may be delayed up to 60 days after painting without sacrificing antifouling performance.
- Boats painted between two and 12 months prior to launch date must be scuff-sanded with 220-grit production paper or abrasive pad before launching.
- Boats painted more than 12 months prior to launch date must be lightly sanded with 100-grit production paper and recoated before launching.
- Boats in the water for less than 24 hours (e.g. for in-the-water water testing) should be pressure washed lightly to remove dirt, salt or other contaminants and allowed to dry. These boats should still be considered newly painted and may be launched up to 60 days after the date of painting.
- Boats in the water for more than 24 hours, but less than 30 days, should be pressure washed when hauled, then lightly sanded with 220-grit production paper immediately before re-launching. If necessary, launching may be delayed up to 60 days after the bottom has been sanded. Note: Boats re-launched within 72 hours of haulout do not need to be sanded before launching.
- Boats in water for more than 30 days should be pressure washed when hauled, lightly sanded with 100-grit production paper and recoated with antifouling paint, even when re-launching will take place within 72 hours.
If there isn’t enought hard bottom paint on the surface it could flake off with sanding due to the thin layer.