First of all it takes quite a lot of training and knowledge to really create fine welded loops. For most (not all) fly lines the biggest percentage of it's breaking strength comes by the core. Mostly it's something like 70% strength by the core and 30% strength by the coating. The coating though doesn't offer much strength to the loop. And then coatings vary in thickness and material.
If you want to weld a coating in order to create a nice loop, you need to test if the particular coating is good for that. With some coatings it's nearly impossible to get proper welded loops. The experts then mix in different sort of coatings in order to get more strength. But believe me you need to know what exactly to mix up/melt together here!
Still all expert welded loops I have fished or seen being fished by my clients did not have the strength of a spliced loop nor did any of them rest nearly the same time. What I have seen instead are several big fish getting lost on broken welded loops! Make sure once the loop starts to get loose you need to change it.
As long as my fly line offers a dacron kind of core I by far prefer to splice my loops for a) a much longer life, b) significant more strength and c) being thinner matching ring diameter much better.
You may find the step by step instruction for splicing a loop below. There are many ways how to do it - that's my way.
One last thought on welded loops: If you use them, you may want to have a first aid solution how to create a new loop out in the field! ;)
My week was loaded with fly fishing as usual. In the weekend I was teaching fly casting and in the other days I was out fishing inbetween some ice. ;)
Warm fishing week to all of you!
All my best
Please find below some pictures of last week and the step by step instruction for how I built a loop with the core of a fly line.