bridge floor core repair

I own a 33 ft bertram sportfish. The bridge decking has soft spots that I need to repair right along the back rail on the overhang. Where the railing feet bolt into the deck it seems to have tapped steel plates that the screws screw into that where glassed in. 1 plate seems to have rusted and caused a pimple that lifted and cracked the skin I am figureing that I have to remove the railing and start drilling small inspection holes to find out how far my rot has gotten. Remove the skin and remove the rotted core.. In doing the repair what kind of support or materials will I need to do this job, ie fiberglass mat requirements type of core, preping the area and supporting the area ex.... Does anyone have any suggestions and recommendations, Thanks, Mike