The submersed part of a vessel should always be carefully observed. The question of whether in a GRP hull an osmosis treatment, or prevention, needs to be carried out can only be answered by close examination. Should the old anti-fouling layers be too thick, or if flaking has begun, it is prudent to remove the old coating and prepare a completely new coat.
In an ideal world the previous coat would only need to be sanded and/or cleaned. Another option available to us is the application of a blocking primer prior to the new antifouling paint.
We can advise you of the available options based on your specific needs.