Please Explain Weld Thru Primer

Ive tried it. Seems to clog up the tip in the can within the first 2 seconds or so. Tried shaking it, turn upside down, swearing, then gave up and welded the part thinking ill use seam sealer and primer/sealer/ base coat/ clear coat, to keep out moisture. The brush on version i hear is probably better as far as application. ...