In a previous life I was Managing Director of Turco Products Ltd. manufacturers of etchants and maskants for the aerospace industry. A process we called 'chemical milling'.
Googling Turco and Chemical Milling will bring up a fair bit of information for you Neil.
The etchants had a basic ingredient of caustic soda with a number of additives. (In a similar way a plating bath has additives to modify the basic chemistry for a better finish). The maskants were made from a mixture of toluene, rubber and chalk.
Can I second the warning to keep well clear of hydrofluoric acid. The pills referred to are to replace the calcium that this leached from your bones and we instructed staff to start taking them straight away if they came into contact with the acid. Nasty stuff. The rare times we used this in our manufacturing process the operatives had to wear full haz mat protection.