If you have stubborn stains on your PVCu window sills which you are having a problem removing try using: Star brite Rust Stain Remover. This Rust Stain Remover can be purchased at most Boat Chandler Stores and is marketed for marine use and it does exactly what it says on the label.
Just simply spray the liquid on the stained sill and let it stand for approximately 5 minutes then wipe off, if the stains are still visible try again and if the stains are ultra-stubborn try a very light rubbing action with a microfiber cloth.
It is very important to wash down the sill after with water in order to remove the residual cleaner.
Finish by drying the sill and finally polish with a couple of coats of PVC Cleaner (not solvent based).
Important notes: Follow the instructions on the bottle and use protective clothing, gloves and eye protection, avoid contact with skin and keep away from children and animals.
Always test a small sample area before using and avoid splashing surrounding painted areas which could be damaged.