A lot of time a dense foam is used for the superstructure pieces. It's lightweight (because on the smaller ships weight can really be an issue), can be painted, and resists BBs.
You're going to need to be more specific, in fact, I would suggest you even ask on the missouri thread about it.
He is probably referring to Stokamoto's NC build He uses foam mat in two places here, first on the bottom of the hull to build up the waterchannel, and second, he used the not very useful otherwise interlocking edge scraps to hold his internal armor in place. I can't say I've ever seen internal armor held in that way, but it looks like it works well and uses scrap material that otherwise wouldn't get used. (You cant see much of where he used it in the waterchannel here, but from OP's comments I figured he was more interested in the pieces holding the shielding in place)
