Software product lines (SPL) are a well-known methodology to develop industry-size adaptable software systems. SPL are often used in domains where high-quality software is desirable, but the overwhelming product diversity remains a challenge for assuring correctness. We present our work in progress on reducing the deductive verification effort across different products of an SPL. On the specification side, our approach enriches the delta language for describing SPLs with formal product specifications. On the verification side, we combine program slicing and similarity-guided proof reuse to ease the verification process.