I have never tried valve grinding compound. What I have tried is toothpaste, different grades of rubbing compound and metal polishes. If I could make any improvement at all it usually wasn’t successful as to getting them totally dry so a motor could sit overnight. I have given up trying. As they are harder to get I have picked up both 45 & 90 deg. petcocks. The ones for 1/4od tubing, that I need for my larger motors, are almost impossible to find. I knock out the pin in the stem and drill out the threads of the thumb screw the size of the stem. Then solder the thumb screw on the stem. Not necessary but I sand off the solder till its flush with the thumb screw. I also will make up a fuel line for the replacement. Put the original line and petcock back on when I’m done running the motor. Tubs
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.