Understanding the temperature dependence of thermal boundary resistance, or Kapitza resistance, between liquid helium and sintered metal has posed a problem in low temperature physics for decades. In the ballistic regime of superfluid 3He-B, we find the Kapitza resistance can be described via scattering of thermal excitations (quasiparticles) with a macroscopic geometric area, rather than the sintered metal's microscopic area. We estimate that a quasiparticle needs on the order of 1000 collisions to successfully thermalise with the sinter. Finally, we find that the Kapitza resistance is approximately doubled with the addition of two mono-layers of solid 4He on the sinter surface, which we attribute to an extra magnetic channel of heat transfer being closed as the non-magnetic solid 4He replaces the magnetic solid 3He.