We report a phase inversion polymer coating to prevent clay swelling and fines production in coal seam gas (coal bed methane) or other petroleum well bores. Importantly, this coating remains permeable to gas and water flow across producing coal seams. This approach uses a water-soluble solvent that can form a low-porosity, dense membrane across clay-containing interburden layers, which contain less free water than the coal matrix, but form a porous membrane across the coal layer. Problematic interburden samples and reference coal samples were from a core sample obtained from a coal seam gas well in the Surat Basin, Queensland. The mixture of polymer coating solution (polyethersulfone (PES)/N-methyl-2-pyrrolidinone (NMP)) was prepared and cast onto the target samples by a simple dip method, followed by the phase-inversion step. Polymer coatings were characterised by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. The SEM images of coated mudstone and coated coal samples showed that the polymer formed a dense layer across the low-permeability mudstone, but an open porous structure was formed on the coal surface. The uncoated rock samples and polymer coated samples were exposed to a flow of distilled water, and the formation of any fine particles was observed by video camera. The flow tests show that the PES coating can significantly improve the stability of interburden samples containing swellable clays.