Great stuff is an open cell insulation and will cause rust issues.You really need to use something that will bond with paint the wont absorb moisture,I'm with ya on sound deadening,i plan on using a closed cell polyolafin sheet insulation(3'x4' sheets) for the inside(free since its one of the products i rep)but the trunk I am not sure,I would probably stick to spraying or something you an slosh around the inside of the trunk lid.I think I am going to try gas tank sealer,there is no easy way to get in there unless you split the trunk lid,which i wouldnt want to do that.The thing i dont like about dynomat is it can absorb moisture.You might even try thinning a truck bed coating and slosh on the inside of the lid,I may try that also since its just in primer at this point of my resto.