Just above the line of antifoul, a green mould develops on the gelcoat, which is extremely hard to brush off. I have been spraying hydrochloric acid on the mould and quickly brushing it off before rinsing with sea water. I had found this to be very effective in cleaning off the green mould. I have recently learnt that hydrochloric acid is not the best product to use, because if left on too long, it can damage the gelcoat, and that oxalic acid is a safer product to use on gelcoat. In the future, when cleaning the waterline, I will be using oxalic acid.

My shore dinghy needed a good cleanup, because the hull quickly gets covered in green mould. I was using hydrochloric acid for this, but will now use oxalic acid.

Green mould on the hull of the shore dinhy

All clean and free of green mould