I would NEVER NEVER put a polyester filler (std filler) over a weld joint.
Spraying epoxy first does not seal out the pinholes.
We always used Duraglass type products until we started having comebacks about 15 years ago. We switched to All Metal type products and have not had a single comeback related to this product. I still see some of the cars I did 20 years ago and can tell the ones I used Duraglass on.
This was not the result of just one can of Duraglass type products.
If you plan to use epoxy first, rad the tech sheet to see if you can put filler over it. Some epoxies do not recommend it. A DTM is not the same as an epoxy.
I usually finish all the filler over bare metal unless there will be an extended repair time, then I will epoxy first. Many times I have used All Metal, rough ground or sanded it close then used epoxy. This allows me to get the body work close, seal it good with epoxy, then I can take my time doing the final filler work. This can be handy in humid weather when you are working on the car part time.