Chapter 9- Body fillers and sanding

Ahhh, the dreaded bondo. Fillers are probably the most important aspect of bodywork, yet, it also happens to be my least favorite. It is also something that is almost an art in itself and requires plenty of practice before you get it right and flawless. Don't let that intimidate you though, as if you sand too much off, you can always easily come back and put more on.

First, I'd like to talk about the different types of fillers:


Fiberglass- These are the most difficult to form and sand as they are the toughest of the fillers. "Kitty hair" as it's sometimes called, is basically a thick fiberglass resin with strands of glass mixed in. There's varying levels of this. Some has more FG strands than the other. The thicker the stuff is, the harder it is to work with and sand, but the stouter and sturdier it is. The thick FG stuff works well for large deep dents or even filling small rust holes (after the rust is cut away, of course). The thinner FG strand stuff is more for final forming to get a good foundation for your finishing fillers. I would use this stuff over a weld, or in help forming custom body mods or FG enclosures. Many times I won't waste too much time sanding FG fillers. I've even been known to use a grinder to rough in the shape close enough to topcoat with finishing fillers.

Plastic- This is what people typically call "bondo". Personally, I will not use Bondo brand plastic filler, as I feel it's very hard to sand and work with. However, I actually recommend it for the beginner for two reasons: One, it's readily available at parts stores and even places like Wal Mart that is open 24 hours and is generally the chapest of the plastic fillers. Second, as a novice, you will likely be using alot of it since you will be spreading it, sanding it, realizing you messed it up and then spreading more a few times before you get the hang of it. Also, since it is a little bit harder to sand, if you learn how to mud and sand with Bondo, once you know what youre doing, you can step up to the better brands and find it to be easier and end up with better results. I am a big fan of USC and Evercoat products. Evercoat's Gold Rage Extreme is my favorite of them all, but it is also expensive- it tends to be double the price of Bondo brand filler. These fillers work well for finishing off a foundation made with FG fillers, and also good for filling dents, dings, scratches and other minor contour repairs. One thing to remember- Plastic fillers are not near as sturdy as FG fillers, so it is recommended that you never have your plastic filler any deeper than 1/8" thick. While that may be an unrealistic expectation sometimes, it is always good to adhere to it as best you can. Once you know what you can get away with, you may be able to risk going a little thicker, but it needs to be in the right application, and even then I'd never go more than 1/4" thick.

Polyester- Of course, this is a form of plastic, but it deserves it's own section. Often referred to as "glaze" or "putty", these are truly finishing fillers, often very runny and not too thick, but very easy to sand. USC makes one called Metal Glaze as well as another called Icing. The Metal Glaze is a little thicker than the Icing, and works very well for filling deep course grit sanding scratches, as well as small dents, dings and chips. The Icing is a little runnier, and doesn't work very well for filling dents, but is excellent for filling sanding scratches, chips, and pits that you may have in your plastic filler. There is a putty made by Bondo that requires no mixing. I'm not sure what it's made of, but I try to avoid it. I've seen it shrink over time and also develop cracks and adhesion problems.


MIXING:

Most body fillers are a two part system. you have the filler itself, and a hardener. By mixing the two, a chemical reaction occurs that catalyzes the filler by a transfer of heat. It's called thermosetting for the science types out there. It's a fairly simple process. Sticking with the manufacturer's mixing ration is a great start. Adding a little more hardener above and beyond what's recommended will help the filler cure quicker, but too much will also make the filler brittle and susceptible to cracking or overall failure. On the flip side, by adding less hardener you can give yourself more time to spread the filler, but too little hardener won't allow the filler to cure all the way and will lead to overall failure down the road.

When mixing your filler, always use a spreader to "spread" the components together. Stirring the filler and hardener will mix it, but it will also allow air pockets to develop inside the filler, which will show itself as pits when you get to sanding. Spreading it together on a mixing board, smooth piece of wood, or metal work well. You can mix on cardboard too, but be careful of the cardboard that's kinda "fuzzy". Anything you mix on that is made of fibers has the ability to shed some of those fibers into your filler, which can result in your filler not coming out as smooth as you'd like. It won't cause the filler to fail, but you may end up with a chunk of cardboard on the surface once you are finished sanding. That would require you to make sure you get those fibers out and smooth the surface.if you left it and primered over it, it will likely show up as a dimple on your final product. Maybe not right away, but eventually it will deteriorate and create a void. I like using the bondo boards that have the tear-off sheets of a coated paper.




   Your choice in fillers really can make a difference in the final product. Maybe not so much in the novice stages, but once you have the practice and experience you will want to experiment with different kinds. You will also find that different fillers work well with different applications. I usually keep one gallon of thick FG filler, one gallon of the finer FG filler, a gallon of regular Evercoat plastic filler, a gallon of Gold Rage Extreme, and a quart each of Metal Glaze and Icing. I "rough" in my repairs with the FG fillers, "semi" the repair with the standard plastic filler and sometimes the Rage. Then "finish" the repair with the Rage and glazes.

Spreading and contouring filler is another thing that takes practice. There's also many ways to go about it. Some people try to fill just the minimal amount they have to in order to fill whatever ding or low spot they are working with, and try to just do a minimal amount of sanding to finish it out. If you are good, this might be all you need. Some people put 3X as much filler as they need and then file or sand off the majority of it. I like to find a happy medium.


   First, you will want to start with a well sanded or ground repair area. Just like with fiberglass work, body fillers need something to "bite" into. I like to start off with a well ground repair area, and the surrounding areas outside of the repair with no finer than 80 grit paper. Basically, if you lay filler over smooth paint or any other area that isn't scuffed, there's nothing for the filler to bite into, and it will fall out eventually- more than likely 6 months after you finish and paint your car. Then you'll wish you'd taken the extra 30 seconds to sand that area first, while it's taking you hours of re-doing it and having to repaint stuff.


SPREADING:

   There are many different types of spreaders both in size and what they are made of. The cheap plastic spreaders are your best bet. the metal spreaders work better for more intricate or detailed spreading, or if you are trying to spread over smaller dents and trying to use as little filler as possible. I use the plastic ones the majority of the time. It's really difficult to tell someone how to spread. It's one of those things where there are basics to follow, but most of it comes from getting in there and doing it and learning what does and doesn't work for you. Like I said earlier, I always try to put on more than I need and file or "cheese grate" it off first to get it close before sanding. I usually put it on about a 1/4" thicker than the actual surface I'm trying to achieve. Once you get a little practice, you'll find ways to improvise spreaders too. I sometimes make shaped ones out of a standard plastic spreaders or pieces of wood. Works good for getting areas with intricate contours. I've also been known to take an 18" long sanding block, cover it in tape and use it as a spreader for large flat areas.


CURING:

   Depending on the amount of hardener you use, the filler will start to get warm and harden up in 2-3 minutes with alot of hardener, or as long as 10-12 with a little hardener. If it's curing faster or longer than that, you will want to remove that batch of filler from the car, and use less or more hardener to achieve the proper curing time frame. You will know its curing when it gets harder to spread and starts balling up while you try to spread it. There are some things to take into consideration during this curing time. As it first begins to cure, after it's lost it's spreadabilty, you will not want to mess with it. It's still a little smooshy but is also trying to bond together. At this point it is forming its own layer but has not adhered itself to the panel below yet. At this point it could all still be easily scraped off the panel. About a 1/4 of the way thru the curing time, the filler begins to interlock with the panel below, but it still fairly pliable to the touch. Now is a good time to take a razor blade, knife or pick to take off any excess filler that may have gotten into panel gaps, wheel well openings or any other nook and cranny that got unwanted filler in it, as the filler is easily cut and manipulated here. The last half of the curing stage you'll notice the filler is not very pliable at all. There will still be heat in it but it will get harder and harder by the minute. This is the time you will want to get to shaping it.


SHAPING:

The best way to know that your filler is ready to shape, is to try and lightly scrape the surface with your fingernail. If it pushes in and makes a divot, it is not cured enough. If it is hard and your nail leave a whitish scratch looking area, it's ready. I like to use a body filler file or "cheese grater" as it's sometimes called, and start grating off the excess filler and roughing in the contours. I'll use the grater until I get real close to the surface I want- I leave about a 1/16" extra on the whole area. At this point you have two options, and what you choose will depend on your own personal finance to labor ratio. If you are saving cash and don't mind the work in sanding later, you will stop here and wait for the filler to cure completely. If you are like me and lazy, you will grab some 40grit paper and give it a rough sanding while it's in it's final 10% of cure time. At this point, the filler is still a wee bit soft and starting to not be so hot. Sanding it at this point will remove the filler very quickly, but also load up your sandpaper as it's very slightly gummy yet. It's like this- I can use two pieces of 40 grit and 2-5 minutes sanding it gummy, or I can come back when it's fully cured and use 1 piece of 40 grit but spend 10-15 minutes sanding it.


First, I'd like to touch on the different types and grits of paper. There are dry sanding papers and wetsanding papers, which I'll talk about more in the primer and prepping Chapters. For fiberglass and filler sanding, you'll use the dry papers.

   You'll use different papers for different tasks, but I use 24-40 grits for surface prep before filler and also for rough shaping of fillers. Then I'll move to 80-180 grits for working the finishing fillers, and may use up to 320grit depending on what I'm doing.




   You will want to use a sanding block of some sort. I have many types I use depending on what I am sanding, but your selection of blocks will depend on your own financial situation. I have a Dura Block set that has blocks from 4"- 24" long. It also comes with a round block for sanding in a radius. I also have some softer foam blocks too, that are shaped in different ways to get into different contours. You can even improvise your own blocks. I've used paintsticks , wood dowels, sockets, anything that would help me achieve the shape I desire. You will want to use some paper with some sort of support other than your hand. If you sand with paper just on your hand, you will sand ruts in the filler from the contours of your fingers. There are times when you can't use a sanding block at all, but you can still have a supported paper. I'll get a strip of sandpaper and wrap it around itself a few times and then use that by hand. That way the paper is sturdy enough to not let your fingers sand ruts, yet flexible enough to get the areas the block can't.

   You will also want to use a "guide coat" if you can. This is basically just a spray paint that you mist over the area you will be sanding. It is designed to not gum up the sandpaper as you sand and works well for determining what's going on as you are sanding. If you don't buy the actual guide coat stuff, you can mist regular spray paint in it's place, but I recommend using flat colors over glossy, as glossy spray paints tend to gum up sandpaper pretty bad. Here's how the guide coat works: Once you spray on a light mist and let it dry a bit, you begin to sand your filler.....




 You'll notice that most of the guide coat sands away quickly, leaving it only in the low spots.  It'll show you not only what dents may need more filling, but will also show sanding scratches from previous grits....




 You pretty much block the filler down until there is no more guide coat, or metal begins to show thru....



 If the metal begins to show thru, you will need to stop and assess a couple things. Use your flat hand to run over the repair area. Does the metal area feel higher than the area around the repair area? If so, you may have a little of the panel that is too far out from when you were doing your metal work. For this you will want to tap that back down a bit with a body hammer until it is to original contour or below. If the metal is of the original contour, then you will need to stop and fill more in the low areas that are still showing guide coat. When refilling I always try to spread it over the entire repair area. Lightly in the areas that don't really need it, then heavier where it does. Sometimes if you try to fill just the low areas, when you come back and sand it, you end up sanding the surrounding filler that was at the contour you wanted, and undercutting it.




   When sanding, I always try to sand in an "X" pattern where I can. Here, the guide coat shows you how the sanding scratches cross each other....



 If you always sand the same way, you can end up puting sanding scratches in fairly deep. I'll make 5-10 strokes one way, then come back and do 5-10 more perpendicular to the previous 5-10. As you sand, you'll be able to see the direction of the sanding scratches, and if you X them, you'll be able to better see how the filler is sanding. If you are in an area where you can really only go one way easily, I at least try to come back and sand across it before coming back with a higher grit. There's also a few tips for sanding body lines and major contours. I try to avoid sanding along body lines or a radius- I always try to sand against it. If you sand along them you run the risk if flattening out parts of the line or radius. Also, running tape along a body line will help keep them straight. Once the tape is on, sand up to the edge of the tape until all the guide coat is gone. Then you can come back and retape the line to the other side and then sand up to that from the other side of the body line. You will also want to try and use the largest blocks and strokes you can. Short choppy strokes with a small block can give you a slightly wavy finish, while long strokes with a long block will give you a larger, smoother area.



Featheredging- When sanding anything, it is always important to understand this. Lets say you have a dime sized chip. Paint and primer may only be .025" thick, but those edges can not only be felt by your fingers, but also be very visible if you painted over it as is. Even if you took some sandpaper and knocked down the edges just enough to make a transition, you would still be able to feel the low spot with your hand, even if you couldn't feel the edges anymore. This is something that will still show up in the paint. What you need to do is featheredge it out and make the transition much greater. You'll begin to see as you sand it out that the different layers will start to show (clear, base, primers, etc...). If I had a dime sized chip, I'd probably sand it out until it looked like a softball sized target with rings about 1/2"+ wide representing each layer. That makes for a much greater transition and shouldn't be visible in the final paint at all. You would also want to do this to any scratches. I like to featheredge my paint out around my repair area before I even spread fillers, but you can also do it as you're sanding your fillers. Of course, featheredging the fillers is highly important as well. If you have an edge of some filler that doesn't make a smooth, gradual transition to the surrounding area, it will show up in the final product. Notice how you can see paint and primer layers and all body filler areas have been thoroughly smoothed out and no hard lines left behind...









During this stage, your hand is your best tool. You will be able to feel stuff with your hand that you can't see with your eyes during the repair stages. Of course, after the car is painted, your eyes are the best tool, so if you can feel something with your hand that just doesn't feel right, then it WILL show up after paint. I've had a few times when I work an area that feels perfect by hand, but then have a slight wave that's visible after the car is painted and shiny.


Here's kinda a step by step of how I'd do a large repair area...

-Grind area needing filler
-Featheredge the paint back into the undamaged area of the panel with a DA sander and 80grit
-Fill with a fiberglass filler to get major low spots filled or cover any welds
-Cheese grate or grind fiberglass filler close
-Fill with a plastic filler over the entire repair area
-Rough in with a cheese grater or 40grit paper
-Guide coat and block with 80grit
-Fill with Rage over the entire repair area
-Guide coat and block with 80 grit.
-Guide coat and block with 180 grit
-Fill with Metal Glaze or Icing
-Guide coat and block with 180grit
-Guide coat and block with 320grit if not planning on using a filler primer, which we'll get into next.....



Chapter 10- Primers, sealers, and finish sanding