Example 3D printing methods involve rapid liquid filling of one or more cavities within a hollow and/or porous 3D printed object. In some examples, a conventional applicator computer-controllably dispenses significantly viscous solidifiable fluid in layers to build up a side wall of the object, and that same applicator or another applicator discharges a second solidifiable fluid at relatively low viscosity to rapidly fill the cavity(s), voids and/or porosity defined by the accurately printed side wall. In some examples, the liquid fill solidifies to permanently embed an internal object (e.g., wire or fiberglass mesh, Kevlar fabric, acrylic, bullet resistant armor, structural reinforcing material, etc.). In some examples, the liquid fill material permanently bonds to the accurately printed side wall. In some examples, the liquid fill material shrinks upon solidifying to create beneficial residual compressive stress within the 3D printed side wall and/or within the solidified material itself. In some examples, the liquid fill material does not solidify and is non-Newtonian to improve impact resistance.