Isotopic purification of He4 is conventionally accomplished by 'filtration' through a superleak which may, for example, be of compressed rouge or of Vicor glass. An intrinsic limitation of this procedure is that a small fraction of the He3 isotopic impurities passes through along with the superfluid He4, the exact proportion depending on the pore size of the superleak itself. In this note we describe a simple refinement of the technique, by which samples of very much higher purity may be prepared and in which the quality of the superleak is relatively unimportant. Our method exploits the well-known phenomenon of heat flush to remove He3 atoms from the vicinity of the superleak.