Perturbations in the Earth's rotation rate at decadal time periods strongly favor the existence of dissipative coupling at the Core--Mantle Boundary (CMB). Here, we explored the plausibility of maintaining a conducting layer on the mantle-side of the CMB, which can couple the outer core and mantle through Lorentz torques. We propose a suction mechanism that maintains a porous medium on the mantle side of the CMB, with the interconnected pore-space partly or entirely filled with liquid iron up to a thickness of ~1 km. The suction arises from the deviatoric stresses supported by the mantle-solid in regions of mantle downwelling. Infiltration of liquid iron occurs by percolation, but is inhibited by the rate of viscous dilation of the solid mantle. Our model enables core-mantle material exchange, and maintains a thin conducting layer that has seismic detection potential. Our model is only marginally satisfactory in explaining the inferred CMB coupling.