A good ground connection is necessary to making a good weld. Ground clamps that come with welders from the factory are often barely adequate, and they usually deteriorate over time. Making a better ground can really boost the performance of any welder. See this Instructable
for how I made improvements to a very common style of grounding clamp. Read the comment section, too, for some update information.
If the finish on the parent metal is not something very special, it is possible to grind away a bare area and attach the ground clamp there. A simple way to get a good ground through heavily painted or plastic coated surfaces without grinding and unnecessarily marring them is to attach a Vise-Grip locking plier
very tightly and then connect the ground clamp to the pliers rather than to the work piece.