Interesting comments, Warren. (an, BTW, the finished job looks stunning!)
I built one of my very first models with 70 degree solder. It still holds together, and being a wagon was hardly the painting challenge presented by this loco, but by the time I'd moved in to my second kit I was using the normal range of solders I hope we all use now. What amazes me is that someone could have built such a wonderful model whilst relying on a fundamentally poor building technique.
I also thought that paste flux was the bees knees. In fairness, with high temp solders it seems to be OK, if messy to clean up afterwards. However, in combination with low temp solders it appears to not boil off properly and I had a problem with white metal parts falling off. I stopped using it! The safety flux, championed by Mickoo, seems to be the ideal combination with low temp solders.
Brian