Compression

The best way to clean valves is glass bead blasting them. Makes them look like new. If you don't have access to that, then a wire wheel on a bench grinder will do....but take the necessary safety precautions.....safety glasses and gloves. They have a nasty reputation for letting the wire wheel snatch them outta your hands. Is this engine going to get a complete rebuild? If not, you're just askin for trouble.