Hydrochloric acid. It's the very best, and you only need a cup full. Not now sold to the great unwashed, but just get a tradesman to get you some.
Hardware stores stock the gentler version, called Spirit of Salts, but you might need two bottles (about £3.00 each when a bought it a few months ago).
Both are very nasty to use, stink to high heaven, but get the job done once and for all.
A super-quick, easy dinner for Gdcs when you’ve all been out all day and they’re ‘starving’….


