I suggested in the other thread where Hockeypuck asked that black flux was the one to get. The two reasons are that it will stay active for longer or if the part is overheated and it also works with stainless steel. These are the same properties that the UK Tenacity No5 or HT5 flux have and I always suggest them for people getting into fabrication of model parts. It also saves having to buy the black stuff when you have already bought the white when you come to want to solder stainless for things like engine exhausts. Although I have the equivelent to the Harris white flux I seldom use it and tend to reach for the HT5
https://www.homemodelenginemachinist.com/threads/v-twin-maybe-v-4.34267/post-414567
https://www.homemodelenginemachinist.com/threads/v-twin-maybe-v-4.34267/post-414567