Pinholes in rads are usually an indication of another issue, such as with open vented systems incorrect plumbing of the cold feed in relation to the pump, a leak that is continuously being topped up by the f & e tank or an automatic filling valve in sealed systems.
It is generally caused by two things, oxidisation from continuous fresh oxygen being introduced into the system or electrolysis from the many different metals within the system.
Are you having to bleed rads frequently?
Some simple tests can be done to see what is happening. For electrolysis, when venting a rad, hold a carbon monoxide alarm at the rad vent. If it alarms, it is hydrogen, which false alarms a CO cell. A powerflush will be required & a decent inhibitor added which will prevent electrolysis from re-occurring.
If not, then it is air & further investigations are required. Either way, a powerflush should be done & an inhibitor added.