Before I continue, I should stress two things. One, I used to be a bodyman and subsequently a body shop owner before my career change to a journo; and two, everyone has different ways of doing things. The following is the way I’ve done bodywork for almost 30 years, and it’s always worked for me. So while it may not be the way everyone does things, when tackling a full paintjob, I like to have the whole body in bare metal. While this may not be necessary on a modern car with good paint, it makes sense with the 60-plus-year-old sheetmetal I’m dealing with here, as you can never tell what lies under that old paint, or even if that paint will hold up under new paint. The quality of any paint job is dependent on the preparation, so why spend good money on materials, not knowing if the foundation is any good? So … the first order of business was to strip the old paint from the fenders and hood. Sandblasting is always a good way to do this, though it can become expensive, and personally I’ve had too many panels warped and ruined by over-zealous blasters. If you have a good one, stick with them! You could sand the paint using a DA sander, but I quickly found the old paint on my fenders clogged sanding discs (and even softpad grinding discs) and the 10 or so coats of different colors on my hood was taking too long to sand off, so I broke out my heat gun and multi-purpose electric tool from Harbor Freight and scraped the paint off these panels. If you go this route, just take care not to overheat and warp the sheetmetal. With the old paint removed down to the original primer, I then used the DA sander with 40-grit discs to get down to the metal. Again, modern opinions will probably suggest 80-grit, but I like to have a good key for any body filler, which 40-grit provides.
Now we’re ready for filler. Or at least we are once the panels have been blown clean with an air gun and wiped over. On opening the tin of filler, you’ll probably see a honey-like resin on top of the filler. This just means the components have separated during storage, so stir the filler with a paint mixing stick until it achieves consistency throughout. You may also find the filler is thicker at the bottom of the tin at first. While cardboard is often used for mixing filler, it absorbs some of the resin, so use a plastic mixing board. I always fill onto bare metal, never over paint, again as you’re relying on the paint’s adhesion to hold the filler, plus you may get shrink marks around the filler showing through the final topcoat. Modern primers are, however, much improved over their predecessors, and many bodymen will apply filler over these primers, although I’ll admit I did this on one of the doors, as I had applied the primer and knew what was underneath it. I’ll be interested to check for shrinkage marks down the road.