Oh.. the 2 1/2 " measurement was the width of the boards they used for core material. and teak or mahogany would be too oily for the resin to adhere to, so it's a material more like pine. they run the length of the deck between the fiberglas layers of the one piece trunk-cabin-deck molding.
I you have to repair your decks .. i am assuming you have teak as well, you have to get to the deck below to fix any delamination issues, clean it up, and prep it for GLUEING down new teak.. nobody in their right mind puts 10,000,000 holes in a perfectly good deck anymore. Essentially, whatever you do, much of the process I just went through is probably part of what you need to do to rebuild your decks... I used a layer of core to create strength, bring the level of the deck back to where it was when the teak was new, and to make the deck impermeable to any water problems if a fitting leaks or a new hole is pierced in the skin. ( I also got a great insulation solution and with glass decks I can travel the tropics without burning my feet on the teak all the time... have you ever fried eggs on a teak deck beforeo? It CAN be done... and the artificial stuff is just as bad I hear). if you want to send me some pics of your decks I can give you some other feedback on what I see.. the critical element is: how old are they and how much teak is left to "fix".
Quick update: only 10 square feet to go and all the core will be sealed in by at least the first layers of the top skin... hope to be priming and sanding end of the week.