Filler & Smoothing
When wielded properly, body filler isn't something to be ashamed of. Even the very best metal repairs will require very thin layers of filler to bring the body to perfection. After each of the patches and repairs were performed on our Mustang, a layer of 3M Platinum Body Filler was spread across the area and then sanded down to level the low spots with the rest of the sheetmetal. The Platinum Filler is 3M's top-shelf filler and is used for the bulk of building up the repaired areas. It sands and finishes very nicely with a sanding block and 3M 40-grit paper to level with the surrounding metal.

After the treatment of filler to the affected areas, it was further sanded with 3M 80-grit paper after coating with 3M Dry Guide Coat, which is basically extremely fine activated carbon dust. The dust sticks into the pores of the filler primer, so as it is sanded the low spots will show up as dark. 3M Platinum Plus Finishing Glaze was used to fill very minor imperfections on steel and over the Platinum Filler when needed. After repeating the process until all spots are smooth, the complete body was sanded with 3M 150-grit to prepare the surface for priming.

Priming & Block-sanding
The initial bodywork took out much of the issues with our Mustang's body, but it's the priming and subsequent block-sanding that really make all the difference in how straight a car ends up. Since we took our Mustang project down to bare steel and aluminum, we have to start with an etching primer such as DuPont's ChromaPremier Low-VOC Etch Primer. This primer is a two-component system etching primer designed for corrosion protection and adhesion to bare steel, aluminum, and galvanized metal. It's not a stand-alone primer though; it must be followed with a Primer-Surfacer, or Primer-Sealer. Essentially, it's used just to create a uniform base for the build primer that the paint will be laid over.

After allowing it to flash, we immediately followed with three coats of DuPont PremierFiller Urethane Primer Filler. Why three coats? The Filler Primer is used to fill in any heavy scratches created from sanding and also to give us something to block-sand. Much like what was done with the filler over the body repairs and modifications, long blocks are used to sand the primer to find the low spots. Very shallow areas will be equalized by the primer, but some will require a tiny touch more glaze to level them.

To find those low spots, the whole car was coated with another fine layer of 3M Dry Guide Coat. This is the time when a great set of variously shaped sanding blocks comes in handy. We've always relied on Eastwood's catalog of products for good blocks, and here we used 3M 150-grit paper on our blocks at this point.

Be cautious during this sanding as ideally we don’t want to break through to metal again. If it happens, it’s not a big deal since the block-sanding will be followed by another round of primer. In our case, we had enough small break-throughs that we spotted in etch primer to take care of the break-throughs. After that, we re-primed the whole car with filler primer and block-sanded again with 150-grit, followed by 320-grit, then 400-grit wet-sanding to relieve the scratches. Once the primer fully dried, we were ready for paint.