weld through primer isnt really needed for what youre doing. Weld through primer is more for like if you had to replace an entire quarter panel or rocker panel. Its a rustproofing measure you use when its going somewhere hard to get to with the rustproofing stuff later. Some manufacturers dont even want weld through primer used when repairing cars due to they feel it makes a non acceptable weld when complete.
Just weld your stuff in and paint it after.