Rodding Roundtable
Block Sanding
By Carl Brunson

Looking at cars at events this spring, I supprised at how many cars have great engines, good looking chassis, sometimes awesome interiors, but really friendly bodies. You know, the body just keeps waving at you. It looks like if there was a little more time spent block sanding that the body would be straight.

I started watching some young guys in the paint and body biz and realize they mostly replace panels and really don't know how to block sand, so I decided to do a short post on blocking. Folks, the Rambler was kroozn because it's kinda a long post for me. If your not interested in paint and body work skip this post. There is a lot more than you may want to know here.

I've been sanding cars for about 30 years . I don't know if that makes me an expert or just stupid, but I can block sand a car to be super straight. Here is what I hope will be useful information on how I block sand. If you use another method, I'd sure like to know about it. Especially if it's an easier way. In writing this I found out it's a little difficult to explain something I do so much of I don't even have to think about it and I'm finding it's hard to describe movements. I don't have old-timers yet, but I think I have a touch of part-timers, so if you can see I left something out, please jump in and fill in the blanks. I'd really like to hear methods from the Great South land..

Straight cars begin with good body work and it makes the rest of the process easier if you can get the body as straight as possible. I tell people doing a car for the first time to do one panel till it's right and then go to the next panel so it doesn't become overwhelming.

First study the panel. Look at the damage on it. Guess as to how it happened because you need to reverse the damage by pushing, pulling and then the hammer and dolly work of getting the metal as close as possible to the original shape without going too far and stretching the metal and creating a high spot. If the metal was stretched when it was damaged or in the repair work there are two options. Beat the stretched metal lower than the surrounding metal or shrinking it (the preferred method). I'm not going into shrinking in this post because it's about block sanding. The perfect repair is metal finished, using no fillers. Unless you've got some experience working metal it's more than likely you'll stretch the metal trying to" metal finish", but there is only one way to get experience. Because this post is about sanding, I'm not going there either. Next "preferred" method is lead work to fill the low spots. It's slow work for me and expensive. I'm not going there either.

What's next? Polyester body fillers. You've heard it called bondo or plastic. Ok, we'll go there. It's what almost everyone uses and what we have today is very good. I believe that if the old time body men who were melting lead on cars would have had access to today's body fillers, there would have never been a stick of lead soldered on a car. Today's fillers are that good, in my opinion.

OK, all the damaged areas have been worked out and none of the metal is higher than the original panel was, study it again. Look at all the areas that will need filler. Sight down the side, look for low spots in the beauty marks and for any more low spots that could be "bumped" out. Check alignment to the adjacent panels. You'd be amazed at how many cars are painted without fitting the panels first. Align for gaps and anything that goes on your panel should be test fitted now (lights, moldings, fenders, grilles, etc.).

After all that, the panel is ready to prep for filler. If there is paint on it, clean with grease and wax remover. Put on your safety glasses, gloves and dust mask (3M #7048 is one of the better ones). Take a grinder with #24 grit 8 " disc and using light pressure go right to left and left to right with only the top 1/3 of the disc. This action will give a cross hatch pattern to the metal showing the low spots without any grinding marks on them and the high spots show up fast. Bump up any really low spots with a dolly or hammer and tap any high spots down . I grind the area 4" to 5" larger than the area to be filled and clean the low spots with a smaller grinder. An old 8" disc can be cut down with straight cuts on the edges and the points created will get into the smaller low spots. A little sandblaster will also get the paint out of the low spots. What you want is a clean, rough surface for the filler to bond to. Don't touch it with your bare hands! Oils from your hands can ruin the bond. Use air to blow the area clean.

Buy a quality body filler. You'll be thankful later. The cheap stuff is about $10 a gallon and the good stuff $20-25. The cheaper fillers cure rock hard and any money you save will be lost in the extra time and sandpaper you'll use sanding it. Stir it up well, the resins that thin the filler tent to rise to the top. I'd recommend a brand, but what works best for me here in Vegas wasn't what I liked in Illinois. Temperature and humidity differences must be the reason. Find out what the quality body shops in your area are using.

Use a paint stick to dip some filler from the can onto a plastic pallet ($2 from the supply house), add the hardener and gently fold the hardener into the filler with the stick until the filler is almost the same color throughout. Scrape the filler off the stick with your applicator (I like the plastic shrink pack kind) and finish gently folding the hardener into the filler. Why do I say gently? If the hardener is aggressively stirred into the filler air is stirred in too and causes little bubbles in the filler that will make a lot of little pin holes in the surface when it's sanded. Follow the directions on the amount of hardener to use. Too little and it'll cure too slow and perhaps not fully cure till after it's painted over. A bad thing. Too much and and the color from the hardener can bleed through to the top coats. A bad thing, too.

Never reuse the stick you use to mix the filler and hardener with to get more filler from the can. A little hardener transferred off the stick to the can may ruin a whole gallon.

The old timers always said that it should only take 3 coats of filler to finish a repair. Generally it's still true.

  1. First fill coat- to take care of the low spots.
  2. Second contour coat- to give it the correct shape.
  3. Third finish coat- to fill pin holes and imperfections

Fill coats should be applied in the lowest area using some pressure in the first strokes of application to force the filler into the grinding marks for a strong bond. The next strokes can be lighter and cover more area leaving enough filler to be a little higher than the metal. I was taught to never apply filler in little dabs or up and down on the side of a car because it just creates waves, so I always apply filler right to left and there is less "highs and lows" to sand out. Most guys like to use a "cheese grater" to remove the excess filler as it starts to firm up. It's quick and easy, but I don't care for the way it has the tendency to break the filler loose from the metal on the edges. I wait till the filler is firm and sand it with #36 grit until it feels a little low. If some metal high spots show, they are tapped slightly down to create a low spot. Now just quickly sand with #80 grit to take out some of the #36 scratches and be sure all the filler has some sanding marks in it to help bond the next coat.

After every sanding blow off all sanding dust. It makes a mess in the shop, but your next application of filler will get the best bond if the surface is clean.

Sanding the filler to contour is a lot easier if it can be smoothly applied, building it a little higher than the panel. Not too much higher and try to taper the edge of the filler so it make a smooth transition to the metal. Filler is a lot easier to put on than sand off. Start with #36 grit on a sanding block. The larger the area of filler to be sanded, the longer the block should be. My longest is 16". Power sanders are ok to start with, but difficult to get that filler super straight with, so hand blocking is almost required. Adhesive backed sandpaper makes available almost any shape you need for a block. PVC pipe, a stiff section of hose or a piece of wood sanded to a needed contour.

How filler is sanded with a block is one of the things that makes a panel super straight or wavy. Start with #36 (3M #02232) or #40 grit on your block and position yourself so you can sand the length of the filler with some pressure on the block. I start sanding in the center of the filler using a back and forth "X" motion, narrowing the sides of the ">X<". The smaller the block, the narrower the sides of the "X" need to be. Sand horizontally as much as possible, up and down motions create the "friendly look", a panel that waves at you.

"Feeling" your body work or block sanding as your working is important. What looks straight may look like heck with paint on it. Feel the entire repair area by running your hand from left to right and right to left. Then check up and down. Feel often as you sand. On the sides of cars the up and down isn't as critical, it doesn't seem to show unless it's pretty bad. Hoods, roofs and deck lids have to be nice feeling in all directions.

Sand till the surface is level and the edges of the filler are starting to feather to the metal, but it's still a little "high" feeling. Now switch to #80 (3M #02230) on the block (it's nice to have two blocks, 1 for 36 & 1 for 80) and sand till it's smooth and the edges of the filler feather smoothly to the metal. If you hit high spots of metal as your sanding, tap them a little low.

It seems like this area is the hardest for most people so I'm going to give a few methods of getting the filler level and feathered onto the metal. Use a "guide coat". If your close to level, but it just doesn't feel right, use a spray can of LACQUER primer and put a dry coat evenly over the area (include the surrounding metal) and then sand again. The primer will sand off the "highs" first and stay in the" lows" letting you see where the problem is. Switch back to the #36 grit then follow with another coat of filler. The coarser the sand paper the flatter it will cut. Smoother grits will "follow" gradual waves and coarser grits won't. Remember this when block sanding anything: Use the coarse grit as long as you can, then switch to the finer grit to remove the coarse grit scratches.

Here is a secret method that works unbelievably well, but you gotta really want your car straight to use this stuff. IT SMELLS RANK. If you "broke wind" like this stuff smells, you could clear a biker bar in seconds . It is grille brick. The black blocks restaurants use to clean grills. It shapes itself to any shape so sand on an area you want to duplicate and that is the shape it will have to block with. It breaks away fast if the surface is rough, so I use it after smoothing some with #80 grit. Grille brick has a coarse side and a smooth side. The brand name "Disco" seems to be less rank than others. It cost from $15 to $25 a case at restaurant supply houses. If someone unwanted is hanging around, just start sanding with this stuff 'n you'll be alone.

Now we're getting somewhere, your all alone and your body work feels like the dent your fixing was almost never there. Blow the area off well and examine the surface closely. Look for pin holes and small bubbles of air that may have only a thin coat of filler covering them. Test any suspect spots with a knife blade or small screwdriver. If there is a bubble, dig it out. If there is any air pockets trapped in the filler, as soon as the car gets hot in the sun, the air is going to expand and look like heck in the finished paint job.

Finish coats should be applied as smooth as possible and there are several ways to thin the filler for the last coat. Some body men add a little fiberglass resin or pour off some of the resin on a new can of filler to add to the last coat and there is "Plastic Honey"( a can of just filler resin) if you can find it. Thinning fillers for the final coat has been almost replaced with with "finishing putty's". These are two part putties that will stick to almost any sanded surface (including paint), spreads like warm butter and sand easy enough they can be finished with fine sandpaper. Great stuff and there are lots of them.

I haven't used all of them, but the ones I have and don't hesitate to recommend are: Evercoat Metal Glaze #100416. If your going to apply more over metal than filler, this seems to have a better "bite". Evercoat Glaze Coat #100417. Great stuff. In the rust belt if it's only a ding that's needing fixed, the paint is sanded and the dings filled with Glaze Coat. The factory corrosion protection isn't taken away by grinding. Good for finishing filler too. Wurth Polyester Filler Extra Fine: My favorite. Does everything the above two do, but hard to find and a bit pricey. Some of the putties have white hardeners. Try to get one with a colored hardener. It's hard to tell when the hardener is mixed thoroughly with the putty without colored hardener.

Apply the "putty coat" as smooth as possible and extend it out beyond the area you've worked on. Start the sanding with #80 and switch to #150 for finishing then a light sand with some #220. Use a guide coat on the finishing if you need to. But get it right. Primer is the next step and using primer costing up to $200 a gallon to do the work of $20 a gallon filler doesn't make much sense.

The #150 grit is 3M #02596. Pretty neat stuff, it's the same 2 3/4" wide as regular blocking sandpaper but comes on a roll and is adhesive backed too. I believe the grits available on the rolls range from #80 to #400. A neat sanding backing pad is Meguiars # E7200. It's not really a block, it's firm enough to sand straight with and to knock out those stink block scratches with #150, but will bend some for the curved areas. Good for color sanding paper too. Helps level that "peel".

It's primer time..... maybe . I don't like to prime repairs that have had filler on them the same day. Yes, I know that it says 1 hour or so on the can, but there is solvents in the fillers and an overnight dry time can't hurt. BTW, I never have my body work scratches show up later in the paint (called shrinking). All paint around your repair must be feathered out and any chips, scratches or other paint imperfections. You must not be able to feel the paint edge. I use #220 grit and sand the surrounding area with #320. Blow the area off well and if you feel you've been touching the area with your bare hands, use grease and wax remover on the painted area ONLY. Fillers have a habit of soaking up the stuff. Could cause you problems later.

Tape and cover everything you don't want primer on. Today's high build primers are heavy and go everywhere you don't want them. Lots of people don't think about door jambs and inner fenders till it's too late. Don't just cover the tail pipe, do the muffler too. Lay something on the floor. If you over spray it, it's gonna be there a while. Anywhere you think primer might go, it will. AND places you think it couldn't possibly go. Fair warning. Ounce of prevention time.

I've posted about the different types of primers before so I'll not go into them much this time, because this is supposed to be about block sanding.

I will say this: IF YOUR NOT IN A WELL VENTILATED AREA AND HAVE A GOOD RESPIRATOR, TAKE IT TO A SHOP AND LET THEM PRIME IT. The stuff can kill you or make you sick. Ain't no car worth it.

The biggest cause of paint failure is solvent trapping. That's putting on another coat before the last one has dried enough and trapping the thinners underneath. They are going to try to come out sometime. In the hot sun later is a good bet.

The second biggest cause of paint failure is drying times again (we're in a hurry about everything these days). Not letting the primer dry long enough before sanding. Here in the desert I feel safe sanding the next day, but in Illinois if I wanted to sand soon, the infrared heat lamps were on the primer. Temperature and humidity play a big role. Check with the paint rep in your area. The info on the can is just general stuff for ideal conditions.

OK, you've etch primed and have 4 or 5 coats of some good filling fully cured primer on your car, let's block sand. Guide coat with a contrasting color. Something you can really see. Spray can silver works on dark primers and rattle can black for the light colored ones.

I like to "dry" block the first go around (yep, there is more than one). Using #150 grit on the longest block that can be used on the panel. If you "wet" sand use #220. Use the same motions as in sanding the filler and putty. More back and forth than up and down. Remember that filler that felt so perfect? Well, how's it look now that your sanding the guide coat off? Chances are not so perfect. It's difficult to blend filler into the metal. That's why so many straight cars are "skim" coated with a few mills of filler and sanded straight.

Ideally the first block sand will take off all the guide coat and the primer is blown off, re-guide coated and re-blocked with £320, then re-guide coat and wet sanded with # P600. Just seal and paint. That's ideally.

Reality: Block until there is no more guide coat. If some filler shows and there is guide coat around it, keep blocking till the guide coat is gone. Sand down that high spot. Stop sanding if there is metal showing. You'll never sand a metal high spot down. Tap it low and use more finishing putty and re-prime.

Before you re-prime, get another panel ready for 1st primer so you can save a little time by priming two panels at a time.Keep going around the car a panel at a time till it's all in 2nd prime and guide coat.

The second prime can be wet blocked with #320 and finish sanded with #P600. It's unlikely you'll break into filler or hit metal on the second time around, but if you know the drill.

Your paint and bodyman

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