I recently put up some fencing using annular nails I thought would be fine based on a few Toolstation reviews. I should have checked it out properly as most have rusted within days of finishing the job. I painted the fence with cuprinol ducksback and at first it covered the nail heads but didn't last. I've since knocked in the nail heads and coated again, which looked fine for about a day but the rust is showing again. Are there any rust prevention products I can touch up the nail heads with that will then take a coat of the cuprinol? The other thing I've thought of doing is filling in the holes and overpainting, but that feels like a lot of work on around 70 slats.