For a first coat of filler over you're welding, I'd get some USC or evercoat all metal, Which is basically resin & ground up aluminum. Best thing for the rough areas for strength & to keep out moisture. Some of the OLD guys these days use it where they used to use lead filler.Final smooth with a regular grade of evercoat or whatnot. Don't skimp on quality of fillers.
Is that just a flux wire mig you're runnin? If you don't have one , a small rotary stainless wirebrush in a drill to cleanup between welds might help you out. And before final level of grinding/ smoothing , tap down any high spots. A high spot is worse than a low spot for finishing.
I know you didn,t ask for my help, but if I can I,d be glad to.
Joe