Concrete-thin = báo. Anything you can hold up and look through - paper, fabric, slices of meat, pancakes, ice - is báo. Its direct opposite is 厚 (thick): 薄的书 vs 厚的书. Soup or coffee being weak is also báo (汤太薄), and so is cold/aloof manner (他对我很薄).
mistakes
薄 has a second reading 薄 for abstract or literary 'slight / weak / meager': 薄弱 (frail), 单薄 (thin/flimsy), 微薄 (meager), 刻薄 (harsh), 厚此薄彼. A third bò survives only in 薄荷 (mint). Rule of thumb: if it's a physical thin object to báo; if it's a figurative compound to bó.
Grass radical on top - three strokes evoking two sprouts side by side, the standing form of 艸. It supplies meaning: 薄 originally referred to thin reeds and slim plant stalks, then generalised to 'thin, slight, weak' - including 薄荷 (mint), 薄饼 (thin pancake) and 单薄 (flimsy). Same radical anchors a vast plant family.
溥 supplies the sound, drifting tone to báo / bó. Same phonetic stem also feeds 博 (broad), 簿 (booklet) and 搏 (grapple). 溥 itself is 氵 + 尃 'spread water' - broad and thin - which echoes the 'thin' meaning of 薄. Useful both as sound and as a faint semantic flavour.
