Try and do this annually and use the correct antifoul for your vessel. I live by the motto Use Good Products, branded and well known. Old antifoul should always be sanded down to remove loose powder, this is as a result of the active biocides having done their job to keep your boat hull clean, a residue  remains of the former coatings. By sanding it down, you remove any growth there may be and provide the new coating with a good adhesion surface and also keep a few extra kilos off your hull. I would also advise that after several years on layered paint, have your hull sand blasted in order to remove excess unwanted weight, and also to check the condiotion of your hull for grp fractures etc. This is a good time to have  asurvey done if needed, and to also apply a few primer coats to prevent Osmosis. There are several branded and proven products, get in touch with us and we can professionaly advise you.

Remember to always follow guidelines and to allow a minimum of 24 hours between coats to allow solvents to evaporate before you apply the next coat. 2 thin coats are sufficient and better than one thick coat. The use of a primer, if in doubt is always better to use and to make sure the new Antifoul stays on your hull.