Underside Painted or Coated
Short answer: From the factory the underside was painted (semi-gloss black on everything except body-color overspray). Full-underside undercoating was a dealer-installed option, not factory — GM did not offer factory rust-prevention/undercoating until the 1980s.
Two exceptions worth knowing:
- The neatly masked undercoating at each wheel well is factory — used mainly as a sound deadener rather than rust-proofing. Its texture is hard to duplicate, so if it is not rusting underneath, leave it alone.
- Dealer add-ons existed early: one 1968 Sprint came with a dealer undercoating plus a “Ming Paint Treatment.” That undercoating was a yellowish, waxy paraffin (like beeswax) and came off with rags and mineral spirits rather than scraping.
On the GTO, a factory option let you delete the sound deadener and body sealer (“dum dum”) to save a little under 100 lbs — mainly for drag racers.
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Content last modified: June 13, 2026 at 10:06 am
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