Flame polishing is the technique of using a heat source, generally an open flame, to melt and thereby polish the edge of a piece of plastic. For reefing, this is usually most applicable to sumps. If done correctly the finished result resembles an edge that is as clear as the original acrylic.
Why is this a problem?
Since you are melting the acrylic the stress on the material is substantial and permanent. Often one will find that the acrylic will craze, i.e. micro fracture, after time and use. The stress from flame polishing a welded seam generally reduces the seam strength by 50%. One of the frequent causes of split seams we see in the shop are from sumps or tanks that have been flame polished. To make things worse none of these are repairable.
So why if flame polishing is bad are builders and manufacturers doing it?
Simple answer is cost. The time to flame polish a sump or tank can be counted in seconds. The time to properly polish a sump via the proper mechanical means is measured in minutes or hours. If a builder gets behind on a custom sump build or the manufacturer is running behind production timelines the time savings can be significant.
Our custom sumps are never flame polished. With all the time and care spent building our products it would be a waste to rush to finish and ruin our products by flame polishing. So we polish all our sumps and tanks by hand. Like all things in this hobby good things come with time and patience.