OK I did a bit more searching, the thread I linked couple posts ago was not the Nassenheider one. I found this old thread from back in 2015 where I described my experience with using formic acid via Nassenheider units.
Won't run through the whole thing here cos the thread has it, but the basics are it was highly successful, and better than the flash treatments. The acid was dispensed over a 6 week period into the hive, and was at least as successful as synthetic strips.
Problems and my reason for not continuing with this method was over time it dissolved all nails in the hive, the lid, and frame wires if wooden frames were used. However now, when the effectiveness of synthetic strips is coming into doubt, the problem with the nails could be overcome by using resistant nails, might have to do some experimentation to find the right nails.
Anyhow, here's the thread
Started use of the Nassenheider formic acid dispensers today, purchased from Manfred. In the first pic they are being set up, the bottles were pre filled with 60% formic acid before I left home. The next pic shows them with the wick screwed in place onto the bottles, and the bottles in place on t...