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Let the plastic come back to room temperature before you continue working on it. Welded plastic doesn’t take long to cool down, but you can wait as long as you please. Look for the welded plastic to turn solid. If you don’t feel any heat coming off of it, you are ready to work on it.  The best time to fix a weld is before it cools. A good weld looks relatively smooth and consistent. Add more of the welding rod or smooth out the melted plastic with your gun as needed. Set your welding gun aside when you’re done. Remember to put it in a safe spot, such as in a heat-resistant holster, until it cools. Smooth out the rough ridges on the weld to make it look consistent with the rest of the plastic. Apply light pressure to the weld, rubbing the sandpaper back and forth across it. Try to get the weld to look level with the area around it, but be careful to avoid scratching the plastic around it. If you’re looking for a speedier way to do this, use a sanding wheel on a rotary tool. Be careful, though, since plastic is relatively soft and easy to scratch. Switch to finer grits of sandpaper to neaten out the weld. Higher-grit sandpaper is finer and less abrasive, but it can still scratch the plastic if you’re not careful. Rub the weld down like you did with the 120-grit sandpaper until it looks consistent and feels smooth to the touch. Always start with the lower-grit sandpaper. It is coarser, so it wears away more of the plastic. Save the higher-grit sandpaper for finishing.

Summary:
Wait at least 5 minutes for the plastic to cool. Sand the welded joint down with 120-grit sandpaper. Finish the plastic with 180 and 320-grit sandpaper.