CTP  Tech

InTheGarageMedia.com

Two rusty car doors leaning against a white wall
In Uniform Fairness
Block Sanding Tips for First-Round Primer-Surfacer
BY “Rotten” Rodney BaumanPhotography BY THE AUTHOR
F

inal-primed and guidecoated, do these doors look pretty “straight”? As shapely as their designers intended for them to be, there ought to be a better word to describe them. Thinking back, we may’ve touched on that subject once before, but I don’t recall any mention of working with Charles and Dave.

Back in the boat shop, my friend Charles preferred the word “fair.” Back in the body shop, my friend Dave would say “uniform.” Now as it sort of seems to me, either of those words fit better than “straight,” so I’ve adopted those two bits of my coworkers/friends’ terminology.

In an earlier tech story we covered initial steps for achieving uniform fairness in the bodywork process. For our ’55 Chevy second-series build we’re using one original door and one reproduction from Brothers Trucks. Both doors had their own unique issues, but due to rust, dents, and extra holes, the original required a good deal more bodywork.

Our doors (along with other panels) are currently in first-round primer-surfacer. Let’s not go into which brand exactly. Primer-surfacer availability tends to depend on shop geography and we have choices wherever we might be.

Here, our own favored primer-surfacer is “regular-build” urethane. Over steel, polyester filler, and epoxy primer we generally go with three wet coats of reduced primer-surfacer for the initial application.

Particularly prior to skim-coating, polyester filler applications tend to sand at varying rates, and areas where feathered edges meet steel (or epoxy if that’s your MO) require extra caution as undercutting can easily occur.

Now in first-round primer-surfacer we have for the first time a surface that’ll sand absolutely evenly. This is our chance to fine-tune our bodywork even further.

Here at this shop we have a pretty-set routine. Our first-round primer-surfacer will be faired with 120-grit adhesive-backed longboard paper. Sanding blocks and fairing boards will be selected to match the varying shapes of our rather curvaceous Task Force doors.

To ensure flexibility, elasticity, and durability down the road, we’ll keep on sanding till our first-round primer-surfacer is thin enough to just about see through. If we expose a little steel or polyester filler that’s usually OK at this stage. When the doors are hung on their fixtures, masked, and chemically cleaned as necessary, they’ll receive one coat of epoxy primer, followed by two coats of reduced urethane primer-surfacer, which we’ll call final-prime.

From final-prime on, paint prep can be done with small flexible blocks and scuff pads for the most part. There’ll be no further need for the longer fairing boards. With only a couple of homebrew exceptions, most of our blocks, boards, and abrasives are available through Summit Racing Equipment.

Of course there’s more to all of this than can be covered in a single printed tech story. Apart from a randomly pointed index finger, the following photos won’t clearly illustrate the direction of sanding strokes, so know that following each lap of diagonal panel passes, we’re switching up to counter in a crosshatch-type of pattern.

If there’s room for just one more thing I’d stress the importance of light pressure and sharp abrasives. At this crucial final-fairing stage, downward pressure on these doors would push their skins out of their natural shapes. The skins won’t sand accurately that way, and if we don’t remain conscious of that, we’ll never achieve uniform fairness.

On those notes, let’s go on and do this job. It’s just another step along the way toward the type of finish our customers expect these days. Perhaps you’ve noticed, too—the bar only goes higher.

Two unpainted car doors hanging on a stand
1. Here at primetime a yard sale swing set makes a fairly fine fixture. The right door on the left is the original. It’s obviously required more work than our reproduction door.
Hands holding a brand-new Motor Guard WB-1
2. Here’s a brand-new Motor Guard WB-1. Partly for the purpose of illustration, here’s what happens whenever we put a new sanding block into service.
Hands washing the Motor Guard WB-1 in an industrial sink
3. This one’s from Summit, so we don’t have the parts store contamination worries. Even so, it may have still been touched by non-painterly hands. Let’s wash it.
Hands sanding the new Motor Guard WB-1
4. Let’s also not assume that this manufactured block is truly true from the factory. With 120-grit abrasives affixed, we use this slab of aluminum now and then for tune-ups.
Hands sanding the new Motor Guard WB-1
5. As you’ll see as we go, the handle-side of this block is most useful. As we would with any new block, let’s sand the edges so we won’t be carving grooves.
The new Motor Guard WB-1 sitting in a drawer
6. Now that our new block is surgically clean and trued-up as necessary, it’ll have its place among others. We’ll be using it here on this job soon enough.
Two unpainted car doors with no interior panels laying on a stand
7. We’re beginning with the B-sides. In the foreground the reproduction door was pretty easy. The original’s telltale primer spots denote bodywork that’s done.
Man using a tool to blow dust off the unpainted car door
8. Moving onto the doors’ A-sides, we’ll begin by blowing any dust away with clean compressed air. We’re quite confident these doors are otherwise clean.
Man applying a dry guidecoat to the exterior of the unpainted car door that's laying on the stand
9. Today, dry guidecoat is nothing new, but having been around a while, I remember when it was. Anyway, this one is the leading brand and it’s available through Summit.
Blocks and fairing boards laying on a work bench
10. Here on the bench are the blocks and fairing boards we’d imagine we’d need. Sticky-back abrasives over time leave deposits of gooey glue, so we routinely clean as we go.
Hands using a fairing block on the exterior of the car door
11. Sometimes the best-suited blocks and/or fairing boards are where we find them. I like to begin in the concave stretches. With 120-grit affixed, this flexible tube fits the bill here.
Hands using a fairing block on the exterior of the car door
12. This particular fairing tool is a modified Dura-Block AF4404. Yes, it’s from Summit. With 120-grit affixed we’re moving onto the next concave stretch.
Hands using a fairing block on the exterior of the car door
13. Out on the greater expanses we’ll continue with 120-grit on an Adjust Flex Sand (AFS) fairing board. Quite sadly, last we’ve heard these are no longer available.
Man wearing a mask and using a fairing block on the exterior of the car door
14. Here and there as necessary, a shaved (lengthwise in-half) Dura-Block AF4403 is the ideal deal. This is another Summit staple, but we tend to modify on the fly.
Hands using a fairing block on the exterior of the car door
15. So, who remembers the Motor Guard WB-1 we sanitized and tuned-up earlier? Here we’ll use the useful handle portion as it’s the best fit for this particular stretch.
Hands using a fairing block on the exterior of the car door
16. Here the soft side of a Motor Guard SB-1 fits the concave curve. These don’t start out curly but they become that way over time. Here it’s the ticket, from Summit as well.
Hands using a fairing block on the exterior of the car door
17. Once again, with 120-grit abrasives affixed, the Motor Guard WB-1 handle edge fits the form of these stretches.
Hands using a fairing block on the edge of the car door
18. We don’t want primer buildup on vulnerable edges. Here a few passes with 120-grit on a stiff Dura-Block AF4405 scuff pad is chip insurance should an impact ever occur.
Hands using a fairing block on the edge of the car door
19. For remaining edges all the way around, a folded and held-curved section of spent 120-grit does the trick just fine.
Unpainted car door laying on a stand
20. At this point the rougher of two doors (the OE door) is ready for final-prime. We’ve worked hard to get here, but it’s turning out real nice.
Hands using a fairing block on the exterior of the other car door
21. Now as a comparison of sorts, let’s turn our attention to the reproduction door. Steps taken will be the same. Tools used will be the same. Still, this’ll likely take less time.
Man applying glazing to the unpainted car door
22. And sure enough, this one’s done much quicker. Sure, we were all warmed-up, but this door was in better shape to begin with.
Hand sanding the applied glaze
23. Other than this tiny ripple, there hasn’t been much glazing to do. The OE door, which required a polyester skim coat had us chasing pinholes for some time.
Two rusty car doors leaning against a white wall
24. Here’s final-primer, guidecoated. We’re planning to wet sand, so we’ve sprayed a thin lacquer primer mixture that won’t wash away like dry guidecoat might. It’s legal here.
The car doors hanging on a stand, ready to be painted
25. Skipping ahead just a bit or two, let’s testdrive a new pair of fixtures! Wet prepped and ready for sealer, base color, and clear, do these doors look pretty “straight”?
 SOURCES
Brothers Trucks/Holley
(800) 977-BROS
holley.com/brands/brothers_trucks/
Summit Racing Equipment
(800) 230-3030
summitracing.com