Here’s where the heavy lifting begins. Many, many passes of plaster or putty, sanding, and primer. The first step was to coat the entire body in plaster, which is best done the messy way: just plunge your hand into the bucket of plaster and smear it on the body buck. Plaster is much better for filling voids than foam is. Especially the insulation foam that comes in a can. Don’t, under any circumstances, use the canned spray foam. It remains flexible permanently, and keeps slowly expanding over a period of weeks or months. If you use it, as I did, to fill voids, you’ll spend days and days digging it out wherever it reaches the surface, refilling the holes with auto body filler.
I discovered that spackling compound, made for smoothing house walls before painting, works great after the plaster. Plaster has a short working time, and you end up mixing lots of little batches when you’re filling ripples. The spackling compound goes on smoothly, you can work it just about as long as you want, it sands extremely easily, and it sands to a feather edge. I also tried gypsum, but it has the disadvantage of remaining water soluble as it doesn’t cure.
I put on a gallon of Jotun Penguard 2-part enamel filler, then found the only auto-body paint supply shop in town and discovered “sprayable body putty”, so I followed up with a couple of gallons of that, spackling and sanding between coats. About the third coat of sprayable body putty, I noticed that the body buck was swelling badly where it had been in the sun. Uh-oh. It turns out the foam expands and contracts with temperature. After that I kept the car only in the garage, never letting sun touch it. It took 2-3 weeks to fix that mistake, now using auto body filler and a double-action (DA) air-powered sander with 40 grit sandpaper, a great combination for this work.
So when I finally got that mess cleared up, I wasn’t too keen on spraying another coat of primer and potentially distorting the surface again. Instead, I went straight to an extra-hard epoxy used for floors, called Sikafloor. This is a very unusual paint as it’s intended to be used only on horizontal surfaces, where it remains liquid for a long time as it flows to become perfectly flat. I sprayed it on, almost unthinned, an “off label use”, but it worked great for my purposes.