I'm with the others, "weld thru" primer is for, well, "welding thru"
It has VERY,
VERY I mean
V-E-R-Y POOR adhesion qualities. It is for the mating surfaces only. The "pinchweld" area where the edges of your rocker meet at the bottom, JUST where they meet, and will be plug or spot welded, that is what weld thru is for.
And don't use the NAPA, it is way too thick. Get some 3M or better yet, SEM. Use it sparingly and let it dry real well before you try to weld thru it. Be sure to cut off the mig wire with every weld so when you start you have a nice new, sharp tipped wire to spear thru the weld thru.
Brian