Student issues with understanding electric potential and interpreting diagrams were explored in a prior study. The prior study showed equipotential diagram modifications of line thickness and color significantly increased student gaze times at the diagrams without increasing correctness. Students' inattention to electric charge sign and its role in electric potential was a major issue. This study implemented further modifications, based on theories of visual attention and affordance, to electric potential diagrams to increase visual salience of charge sign. Students ranked electric potentials for points on traditional or modified diagrams. Pre- and posttest comparisons and interview results showed training with modified diagrams produced correctness gains of 21% compared with gains of 11% for training with traditional diagrams, and improvement of 36% in application of a conditional rule including charge sign compared with the prior study. In-person training combined with modified diagrams yielded highest pre to post gains of 27%.