Tips to Proper Block-Sanding Techniques

By Barry Kluczyk   –   Photography by the Author

“We might spend 400 hours on the body surface of a restoration project and only 8 of them are actually spent in the spray booth to lay on paint,” restorer Nyle Wing says. “The rest of that time is spent block sanding.”

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Sanding, sanding, and more sanding. It’s the method of smoothing the small surface waves and high and low spots to ensure the panels will be as straight and smooth enough for a mirror-like finish.

“A great paintjob is all about the prep and that’s what we’re talking about here,” Wing, who’s been restoring muscle cars for about 30 years, says. “You can lay on the color flawlessly, but if the surface isn’t perfectly smooth to begin with, you’ll never achieve a show-winning result.”

001 The bodywork comes first whether repairing dents and rust or a complete panel replacement
The bodywork comes first, whether repairing dents and rust or a complete panel replacement, but corrosion develops on bare metal faster than a cop will write you a speeding ticket, so it’s important to get the body in primer as quickly as possible.

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Everything, of course, is relative. The body shop repairing the parking lot scrape on your daily driver won’t spent a tenth of the time on the repair and still deliver an excellent result—but it’s one that reflects and emulates the original production-line paintjob. For a top-tier restoration, it takes more than a couple passes with a DA before hitting the trigger on the spray gun.

002 For this project PPG VP2050 high build primer was used for all stages of paint preparation
For this project PPG VP2050 high-build primer was used for all stages of paint preparation. Some painters will use different primers for different stages, but restorer Nyle Wing prefers using this single product because it works well direct to metal from the start, and there’s no question about how it will react with other products as the project progresses.

In fact, Wing’s shop doesn’t generally use power-sanding tools, preferring the more tactile feel that comes with hand-sanding. It unquestionably adds time—and labor cost—to the project, but he says you can’t argue with the results.

“There are a lot of curves and subtle transitions in the sheetmetal of these cars,” Wing says. “It’s very easy to ‘move’ a body line when sanding, so you really need to feel every contour to make sure you maintain the proper shape and lines. It’s easier to do that using sanding blocks–especially if you don’t have a lot of bodywork experience.”

003 For this project PPG VP2050 high build primer was used for all stages of paint preparation
Different paint guns and tips are used for paint and primer. For primer, restorer Wing uses a Sata gun with a wide, 1.9mm tip. That’s because primer is very thick compared to paint, which only requires something like a 1.3mm tip for its water-like flow properties.

We stopped in recently to look over the shoulders of Wing and his guys as they relentlessly sawed away at the flanks on a vintage A-body. More than just procedure and technique, they advised on the materials and other methods they employ to ensure the proper pre-paint surface finish, including wet sanding the primer for an ultra-smooth surface. And before you fire off a letter pointing out that not every photo is of the same car, we caught the project about midway when we arrived with our cameras, and to illustrate the front and back ends of the story, we had to grab a few supporting images from other vehicles.

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004 With the primer coat protecting the sheetmetal additional bodywork is virtually inevitable
With the primer coat protecting the sheetmetal, additional bodywork is virtually inevitable and filler is used to smooth and flatten the body panels.

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Before even starting, Wing noted that keeping the work area free of contaminants, particularly anything silicone-based, is essentially for a proper finish. The problem is silicone is found in tons of products you probably never considered, including personal-care items such as deodorant and shampoo. Dimethicone is a common ingredient in them, so it’s important to check labels and remember this mantra: If one of the ingredients ends in “cone,” leave it alone.

005 3M’s Platinum Select Filler (PN 31131) is very creamy and spreads very easily according to Wing
3M’s Platinum Select Filler (PN 31131) is very creamy and spreads very easily, according to Wing. 3M claims the formula significantly reduces pinholes and the need for primer. It works well on non-steel components, too, including aluminum as well as the SMC and fiberglass components used on Corvettes.

“Silicone can cause major fish-eye problems, because its chemistry just doesn’t mix with the products used to refinish a car,” Wing says. “And you may not think it would be a big deal but think about how you’re hunched over the vehicle, working up a little sweat while you’re sanding. It’s very easy to transfer some of that product to the vehicle surface. It doesn’t take much to cause a problem, and you’ll be kicking yourself because you’ll have to redo the work.”

For the record, Wing and his employees all wear Old Spice deodorant in the shop, which does not contain silicone.

006 3M’s Platinum Select Filler (PN 31131) is very creamy and spreads very easily according to Wing
After the filler is applied, the first stage of sanding attacks the surface aggressively with 80-grit sandpaper. It’s used to simply knock down the filler and other high spots, getting the surface even for the next sanding stages. Another couple coats of primer are applied at this stage, too, further sealing the sheetmetal.

Proper surface prep takes lots of it, and when it’s done correctly, the results will be unmistakably sharp. Follow the photos and you’ll get a better idea about why those show-winning paintjobs look so good and why they cost so much. The bottom line is time—and plenty of it.

007 the next steps are hitting it with 180 grit sandpaper and again with 220 grit
If the sheetmetal is reasonably straight and smooth after the initial 80-grit stage, the next steps are hitting it with 180-grit sandpaper and again with 220-grit, using a variety of sanding blocks.
008 This is only a partial look at the range of sanding blocks that can be used
This is only a partial look at the range of sanding blocks that can be used. Besides the variety of sizes, a diversity in rigidity suits different panel surfaces. Wing has even made a few super-rigid blocks out of polycarbonate (arrow) to ensure a super-flat surface on long, flat panels.
009 Very small sanding blocks are used for the body’s nooks and crannies
Very small sanding blocks are used for the body’s nooks and crannies, such as the rolled area of the roof that transitions into the driprail. The obvious point here is to use a block that fits the area being worked on, but it’s also important to use one with the greatest possible surface area to keep things flat and smooth.
010 sander for most jobs because he feels it lacks the tactile feel that comes with hand sanding
Wing doesn’t use a dual-action (DA) sander for most jobs because he feels it lacks the tactile feel that comes with hand-sanding. The cushioned pad illustrated here, he says, is a perfect example, because the softness of it makes it difficult to discern body lines and other contours or transitions in the surface.
011 After running over the body with 180 grit sandpaper a guidecoat is applied
After running over the body with 180-grit sandpaper, a guidecoat is applied. As a contrast to the primer below, it will reveal high and low spots below that require further blocking.
012 This graphite based 3M Dry Guide Coat is applied with a pad
This graphite-based 3M Dry Guide Coat is applied with a pad. Compared to many aerosol-type spray coats, it typically penetrates every last crevice and sandpaper hatch mark in the surface with more even coverage.
013 With the guidecoat applied sanding begins again with 220 grit paper
With the guidecoat applied, sanding begins again with 220-grit paper. Aggressive action isn’t required or advised when removing the guidecoat. The idea is to lightly sand it off to reveal the contrast of the surface below it.
014 This close up shot of the roof panel illustrates how the guide reveals not only high (brighter) and low (darker) areas
This close-up shot of the roof panel illustrates how the guide reveals not only high (brighter) and low (darker) areas, but the comparatively course pattern left by the 180-grit paper. It all reinforces the importance of continued blocking with 220-grit to smooth out the surface further.
015 When it comes to sanding technique it’s important to use a crisscross pattern
When it comes to sanding technique, it’s important to use a crisscross or “X” pattern, pushing the block back and forth in each direction. That eliminates the chance of building up unnecessary ridges in the surface or new waves in the sheetmetal.
016 One of Wing’s additional tips is offsetting the paper on the block slightly
One of Wing’s additional tips is offsetting the paper on the block slightly when following a curved contour, such as where the C-pillar sail panel blends with the quarter-panel on cars such as the ’67 Chevelle. It helps the paper more easily follow the natural contour of the sheetmetal, while retaining the pressure on the paper required to hit the surface.
017 For panel edges work the block up to the edge but don’t wrap over it which can dull or round off the edge
For panel edges, work the block up to the edge, but don’t wrap over it, which can dull or round off the edge. It is easy to reshape the panel when sanding, and maintaining sharp, crisp body lines greatly enhances the vehicle’s presentation after painting.
018 applying masking tape along the leading edge of the line provides an excellent guide
For softer body lines, which are even more susceptible to reshaping, applying masking tape along the leading edge of the line provides an excellent guide. You sand up to the tape and not over it.
019 applying masking tape along the leading edge of the line provides an excellent guide
Sanding through to bare metal here revealed a high spot that needed to be knocked down–and that’s entirely the reason for block-sanding. With the area smoothed and flattened, it will be skimmed with a thin layer of finish filler and primed again.
020 USC Icing polyester putty works very well for the minor filling projects
USC Icing polyester putty works very well for the minor filling projects, such as the one illustrated in the previous photo. It has a super-creamy, almost liquid-like consistency that makes it very easy to apply and spread. It goes on thinly and dries quickly, making relatively fast work of minor touch-ups.
021 With the 220 grit sanding completed the body is sprayed with prep solvent to highlight the surface
With the 220-grit sanding completed, the body is sprayed with prep solvent to highlight the surface, allowing reflected light to highlight surface smoothness and, contrastingly, imperfections that still require attention.
022 With the 220 grit sanding completed the body is sprayed with prep solvent to highlight the surface
For this project, PPG Acryli-Clean DX330 was used for every stage of the paint prep process. The only caveat is that it needs to be wiped off before it dries.
023 demonstration on the primer wetted with prep solvent shows the smoothness of the body after 220 grit blocking
This cheeky demonstration on the primer wetted with prep solvent shows the smoothness of the body after 220-grit blocking. Looks good enough to paint, but there are still a couple additional steps before the body is rolled into the spray booth.
024 the primed sheetmetal shows some orange peel and a slight coarseness in the surface
With the prep solvent wiped off, the primed sheetmetal shows some orange peel and a slight coarseness in the surface. It will be wet-sanded to get it that much smoother before the paint’s basecoat is applied–making the final paintjob all the smoother, too.
025 the surface is wet sanded with 320 grit paper and the results are clearly evident here
For that final stage of sanding the primer, the surface is wet-sanded with 320-grit paper and the results are clearly evident here. If the surface is sufficiently smooth after the 320-grit hit, it’s basically ready for paint. If not, it’s another round with 600-grit.
026 After wet sanding the body will be sprayed again with a couple more layers of primer
After wet-sanding, the body will be sprayed again with a couple more layers of primer, reduced more than normal, at 2:1:1. That thins it out more, because there’s no need for a high build for sanding any longer–just a painting surface. Letting the primer cure sufficiently is imperative for a top-notch finish. Like the basecoat and clearcoats to be sprayed later, it will shrink, so it should cure for several days or even a week or two.
027 color sanding delivers the final finish appearance after painting
Of course, color-sanding delivers the final finish appearance after painting, but ensuring the surface was perfect in the first place laid the foundation for that perfect, mile-deep result. The right materials and tools help make it happen, but it all takes time, patience, and attention to detail.

Source:

PPG Automotive Refinish
(800) 647-6050
ppg.com/autocoatings/en-us

Click on this issue’s cover to see the enhanced digital version of Tips to Proper Block-Sanding Techniques.acp april 2024

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