A desmear process for removing resin smeared on an interior wall of a through hole drilled in a resinous substrate, especially resinous substrates made from epoxy, polyimide, cyanate ester resins or bis-maleimide triazine epoxy resins. The process involves contacting the resin smear with a mixture of gamma-butyrolactone and water to soften the resin smear, followed by treatment with an alkaline permanganate solution to remove the softened resin, and treatment with an aqueous acidic neutralizer to neutralize and remove the permanganate residues. The gamma-butyrolactone is effective as a single solvent for softening and swelling resin smears from substrates made from epoxy, polyimide, cyanate ester resins, bis-maleimide triazine epoxy resins, and polyimide resins.