How to get Perfect Body Panels with Block Sanding

Posted: February 16, 2015 By: MattM

How to get Perfect Body Panels with Block Sanding:

With Body Work Expert Kevin Tetz

Ever wonder what it is that separates the mirror smooth bodies of show cars from the body filler fender bender repair you did in your driveway? Often times it all comes down to the important step between the first coat of primer and the first coat of paint, called blocking. Without proper blocking, no matter how good the painter is you are never going to get a perfect show car finish on your project.

Kevin Tetz has probably painted more cars than most of us will ever own, or even drive. He is an expert on high quality finishes, and what you need to do before you start spraying paint in order to get them. His excellent Paintucation series of DVDs covers many aspects of paint and body work, but last year he did a live video demonstration and Q & A all about blocking body panels to get them perfectly smooth before painting.

Blocking 1

Blocking – What does it mean and what is it?

The name blocking comes from the original way it was done – with a piece of sandpaper wrapped around a block of wood. The hard, flat block allowed the paper to sand a flat panel to a much more uniform surface. Later, the heavy rubber style “alligator” or “vampire” sanding blocks were made with several teeth in them to grip a 1/4 strip of sand paper, but that is still dark ages technology compared to today. Now there are blocks of all shapes, sizes and materials, for use on panels of many different contours.

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Blocking is the way you smooth minor surface waves and imperfections, much smaller than dents and dings. These are typically high and low spots so small you really can’t feel them, but you will certainly see them in the wavy reflections of light in a glossy top coat. Kevin drew this magnified cross section of a body panel (the lowest dark wavy line), covered with 2 coats of primer (the other 2 lines).

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Of course, if the substrate you apply primer to has waves in it, the primer will shrink and conform to them as it dries. The next coat will do the same, and so on, and so on, up to the final clear coat. In order to get a smooth as glass top coat you need to solve this while you are still in the primer phase by blocking the entire car. You can see how the large red block will bridge the highs and lows, sanding into the primer until it is perfectly smooth.

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Different Blocks for Different Jobs

Sanding blocks have come a long way from the old piece of wood, or hard rubber 1/4 sheet style. There are long blocks and short blocks, round blocks and square blocks, soft blocks and hard blocks. There are even flexible blocks that can be molded to the curve of a body panel and stay that way until you form a different curve with them.

A good place to start is with a kit with an assortment of rubber block in it like this one from Dura-Block. The large sizes make short work of large flat panels, and the smaller sizes allow you to get into tighter spots and curves. There is also a long skinny block and a round tubular block for various areas and contours. These blocks are made of a rubber similar to the earlier style “alligator” block, with some flex to it, but not a lot. By varying the thickness Dura-Block varies the stiffness and flexibility of the blocks too.

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As you can see there is still some give and flex to them, but they also have a nice sharp leading edge for better cutting. The long straight edge also is great for straightening out character lines stamped into the body work which can lose their focus under too much high build primer.

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The cool thing about these is the stiffness of them is adjustable by inserting up to 3 steel rods in them. That way you can use it to block something flat like a hood, then take out the rods and block the curved transition to the fender without changing tools.

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One of the newest and neatest ways to block are the flexible, lightweight, Soft-Sanders. These are made of different stiffness of foam and different contours, color coded for easy identification. The cool think about these is that they can be bent, then the paper stuck to them, and they will maintain the contour of the bend.

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Clean Up and Safety

Another really useful tool to have around the shop if you are blocking primer and body fillers is a good shop vacuum. Blocking produces lots of fine powdery dust, and it can be hard to get it out of tight spots on body panels without a vacuum. A dedicated shop vacuum is much better for this than ruining your household vacuum and getting your wife mad at you.

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And speaking of dust, it’s a good idea to wear a dust mask to keep from breathing all the powdery, fine dust you are going to be creating as you block the entire car. It doesn’t have to be anything fancy, just your typical dust mask.

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The polyester body fillers and surfacers have a quick cure time and don’t give off any isocyanates. But if you spray 2K urethane primer, that has a 90 day cure time. During that entire time it can be giving off isocyanates that are bad for you. So consider that before blocking and take proper safety precautions.

Primers

For the purposes of blocking and surfacing, you are going to want to be using a high build 2k urethane, or polyester primer. Under that on the bare metal you’d use an epoxy or etching primer, but for this step you need the high build aspect those don’t offer.
2k urethanes are extremely tough, but they are not meant to be built up very thick. If you spray more than 3 or 4 coats they can start to trap solvents in them. Plus, with that 90 day cure window they are going to be shrinking that entire time, so the thinner the better.

2k urethanes do come in handy though, especially in the new catalyzed spray cans for smaller areas and detail work. You just push the button and shake, and now you have a real 2k primer, right out of the can with no compressor or spray gun needed. If you are mixing your own and spraying it out of a gun, a 1.6-1.8mm tip is going to be ideal, and a lot of CFM to atomize those fat particles.

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For surfacing the best is the Contour High Build Polyester Primer. It is extremely high build with almost zero shrinkage. It is very similar to the Contour body filler only in a thinner, sprayable version. It comes out of the gun thick and there is no thinning of reducing, so a 2.0mm spray gun tip is going to be your best bet.

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Lastly, a great way to see what still needs work, and what is perfectly flat is with a guide coat. There are dry rub on guide coats and aerosol spray versions, but they both work the same way. By spraying a contrasting color over the primer, when sanded the guide coat quickly come of the high spots, and stays in the low spots guiding where you need to further block. Guide coat is like no build paint, sanding off with the least effort possible.

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Sanding verses Shaping

Both sanding and shaping are done using sand paper of different grits. Sanding is done to give the paint a good surface to adhere too with 220 and finer, higher numbered grit papers. Shaping is done with rougher paper in the neighborhood of 220 grit and lower and is the final stage of the body work. Sanding is really the first stage of surface prep.
The Sandpaper

All these sanding blocks are designed to be used with rolls of peel and stick sandpaper. These rolls come in a variety of grits, and both wet/dry and dry versions. They are 2 3/4 inch wide, by 82 foot long rolls with peel and stick adhesive on back. Just cut or rip to length, peel and stick to your block. For this first sanding of the primed panel we are starting with a 120 grit.

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Blocking Technique
Perhaps the most important aspect of block sanding a body panel is your technique. You want to work in an X pattern, pushing the broad leading edge of the block back and forth. Doing it this way you avoid sanding new waves into the panel you are trying to smooth.

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Pushing the leading edge cuts and shapes quickly, saving you time. The wide width of the block allows it to bridge over low spots and take the tops off of high spots for a flatter result.

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Work the block up and down and back and forth, crossing over the last stroke in the opposite diagonal direction. Work your way slowly across the whole panel until you have blocked the entire surface several times. Then clean the dust off and examine the guide coat.
You can easily see that the high spots that got sanded are now lighter, almost white. While the low spots still have gray guide coat in them.

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Wet Checking

A great way to check your blocking progress, besides the guide coat, is by wet checking. Simply spray a thin layer of PRE painting prep on the panel to get it wet. Not before the PRE evaporates get a good look at the way light reflects off the panel. This will give you an indication of what it is going to look like after paint and clear coat. You’ll be able to see the waves left in the panel as distortions in the reflection. With the straight fluorescent light it is particularly pronounced.

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Trickier Areas

Unless you are restoring a Volvo or a Plymouth K car, chances are your panels aren’t just large flat areas. This means there are going to be curves to block around, character lines that are going to have to be protected from too much blocking, and lines that need to be cleaned up because of poor body work in the past.
Curves

So how do you sand around a curve with a flat block? The important thing is you want to sand across the face of the curve, like a boat sailing up and over a wave, not alongside of it. If you block along the curve all you are doing is sanding more groves into the panel and changing the shape of the curve itself from the factory contour.

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Work the curve the same way you work the rest of the body panels. If you sand through and hit body filler, or bare metal, let that be a signal to stop and apply another coat of primer.

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Character Lines and Details

Next, we’re going to block along the style line on this fender and show some ways to do it.

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The wrong way to do it is the way we blocked around the curve. If you try to sand an X pattern over the top of the panel and the style line the block will just jump the area near the line, and try to knock down the leading edge of it.

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For these kinds of details you need to sand along the line. Apply a layer of masking tape as if you were masking it for paint, and sand along the edge of the tape with little motions. The tape with help to keep you from sanding into the style line.

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Mask the area you just blocked, and then shape the area that was covered with tape.

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Another way to block a large curved surface is with the Soft-Sander blocks. They are flexible enough to bend to most car panels. But here’s a neat trick with them. Cut a piece of the adhesive sandpaper that is the right size and lay it out on the panel.

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Bend the soft block to conform to the surface while sticking it to the paper.

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The adhesive and the paper will hold the block in the curve you just pressed it into!

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To make it extra rigid you can add a second layer of adhesive paper to the top of it as well.

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With these blocks and the cloth backed adhesive paper, you can peal it off and form it to a new curve, then restick it too. Another great feature of the Soft-Sanders is they are great for sanding complex shapes.

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You can wrap the sandpaper around any one of the edges and get a contour that is going to help you to block the edge of a detail like this style line.

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Question and Answer Time

Why doesn’t Kevin block sand wet? – For Kevin it’s just a matter of preference, and he prefers to keep his feet and shoes dry. On top of that, when working in a collision repair body shop where time is money, he learned by dry sanding it eliminated waiting for the water to dry after wet sanding, so it was faster.

Should I use a guide coat for body filler, before primer? – Yes, you can if you want to. The dry filler is particularly good because you just wipe it on, and the high spots sand off immediately.

What grit do you block down to before final paint? – The answer is it depends. With a solid color you don’t have to block down to as fine a grit as for a metallic or pearl color. The metal particles will line up in tiny scratches so it needs to have a smoother surface to begin with. 400 grit is fine for solids, but 600 or 800 is needed for the fancier stuff.

How many times do you have to prime and block the panel for it to be straight? – There is no real number. That is why you need guide coat, and wet checking. When you no longer see low and high spots you are done and it’s ready for final prep for paint. It may take 4 or more blocking and priming sessions to get a car really show car straight.

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