You might want to save this Tech info. https://www.chevy********.com/tech-stories/paint-body/body-fillers-painting-101/
Very timely as I am currently working my way around my car knocking out dents and getting ready for primer and filler. Thanks Jeff
Here is another site that may be of interest. 3M Automotive Academy Courses. Most are free. I haven't looked at any yet, but plan to in the future. https://www.3m.com/3M/en_US/learning-marketplace/automotive-academy/catalog/
I never was any good at using body filler. I can hang and align body body parts and do the mechanical stuff. Welding, forget it.
Say no to body filler! Metal work takes practice but once you get it, it's very rewarding. It will last longer. Excessive body filler will ruin overtime. The best thing I ever learnt about welding sheet metal was controlling the heat. A wet rag and a bucket water to bring the temperature down.