One obstacle to understanding abstract concepts such as the "control of variables" strategy (CVS) is the tendency for learners to focus on surface rather than deep features in instructional materials. However, in tasks such as learning CVS, these same surface features may also support understanding, provided learners realize the underlying task goal. In this study, we explored the effect of surface features in textually described experiments on middle-school students' understanding of CVS. We investigated whether the amount of surface detail--or surface-level concreteness--of experiments interacts with student tendency to focus on deep or surface features. As predicted, deep focusers showed better posttest performance when given all concrete examples (concrete-only condition) than when subsequent examples became more concrete (abstract-fading condition) or less concrete (concrete-fading condition). Concrete representations helped deep focusers understand the rationale for controlling variables. Although surface focusers who were given only concrete examples showed better understanding on some measures, they generally failed to develop complete explicit understanding of CVS, including its rationale. Consequently, surface focusers showed similarly poor transfer across conditions. Although students generally benefited from concrete representations, surface focusers may need more support to develop sufficiently coherent understandings that facilitate transfer.