Extruded foams (XPS) and moulded blocks often have a denser skin on the outer faces from manufacturing. It can cut slightly differently from the softer core — sometimes needing a touch more heat — and on wings the skin orientation affects strength.
Usually it is not a problem: keep your heat/feed balanced and the wire glides through skin and core alike. If you want a fully uniform core, you can skim the skin off first, but most builders just cut through it. Be aware that the skin is stiffer, so very thin parts that rely on it for strength behave differently once the skin is cut away. Test on an offcut if the finish or stiffness matters.
Usually it is not a problem: keep your heat/feed balanced and the wire glides through skin and core alike. If you want a fully uniform core, you can skim the skin off first, but most builders just cut through it. Be aware that the skin is stiffer, so very thin parts that rely on it for strength behave differently once the skin is cut away. Test on an offcut if the finish or stiffness matters.