A good epoxy works well. I use filler with it sometimes but not always, depends on what I'm putting in place.
Depends on what the mount is for. If it's for a pump, then epoxy is fine, but gun mounts may need something a bit more serious. For these, I fit mounting blocks that are screwed through the bottom of the hull, while the epoxy under them is wet. Then I attatch the cannon to them. Very solid.
DarrenScott, how then do you hide the bolt? Or is it out of sight and out of mind when the ship is in the water so it doesn't matter?
I use stainless countersunk screws, recessed into the bottom of the hull, and faired over with epoxy. This hides them and seals them too.
I use Ca glue to hold down the waterproof foam then I seal the foam with Resin. It prevents the foam from being gouged or frayed when moving equipment around and makes the water run around and over much more smoothly. It secures the foam in place permanently. I've tried this on the Roma and will be doing it for the PE.
