trouble using bondo

Lots of good stuff here and I'll just add a bit since I've been at this so long, it's the internet and I'm entitled :iconbigg:

We used to use a product called Allmetal back in the day for that first coat of filler over the weld. It's not to build but to get the material over the weld. It was (is?) a filler product with aluminum powder in it that inhibits rust. A bit much for most applications but for the fusspot out there it's the way to go. Grind it down well past flush so the next layer of bondo completely covers it. Mix the filler well to eliminate as many air bubbles as you can. Cover as much of the repair as you can-hopefully all of it. Sand in a cross-hatch pattern to get it straight; not up-and-down or along the length. If you need to apply more filler to get it, be sure to reapply over the whole area or you will take filler out of the spots you think are done when you sand the parts that aren't. Finish with at least 80 grit before you prime. I use 180 just to save myself the trouble of wetsanding deep scratches out.

One more tip comes to mind. If you're right handed, check your work with your left hand. It's more sensitive. Vise-versa for lefties.

HTH

Ken (hacking up bondo dust as I type)

And another-the best metal prep for filler (if you can get away with it) is blasting the metal. Grinding scratches the metal and works well enough but blasting puts hundreds of pits in the metal that really grip the filler.