I use epoxy primer, per a restoration publication I read quite a few years back it offers better long term protection. Regardless of what you use, I think you'll find as stated above that weld through is more of a name and less of the process of how it's actually done. Too much around the weld area will contaminate the weld. For cleaning paint out of the plug weld hole, I use a modified bit that's been flattened and backfaced to resemble a milling bit. Works well to clean the paint with minimal "metal removal" from adjacent metal. See here:

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