Final Surface Preparation: Using Body Fillers
In Chapter 3, I discussed how to repair rusted and damaged body panels prior to their final surface preparation and painting. The goal was to restore a solid, relatively smooth surface to work with. High-build/sandable primers and other coatings or sealers sometimes provide the necessary level of smoothness required prior to painting, but this is generally the case for less visible areas, such as under the car or underhood. Larger areas such as main body panels, the hood, trunk lid, etc., generally need additional finishing. This requires using body filler to mask any remaining surface irregularities and to achieve the final desired shape of the panel.
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There are different types of body fillers for different stages of the process, but I will discuss only the regular filler used to cover the damaged area and the glazing/spot putty skim coat that is used to fill in smaller imperfections like pinholes. I will only deal with two-part so-called plastic body fillers that are mixed just prior to application. (I will not get into more exotic fillers such as lead, which require heating with a torch. The skill required with these techniques and materials is beyond what most shops are capable of; their use is gradually becoming a lost art.)
While some may say the use of lead and similar technologies provides a superior result, this has become less true as the technology of conventional fillers has improved. When you consider the potential toxicity of working with lead, plus the additional safety considerations and equipment involved, there generally isn’t a compelling argument to use it for anything but authenticity. That’s not likely to be a concern with first-generation Mustangs.
After the underlying material (metal or otherwise) is as close to the final shape as possible, clean it before applying any type of body filler. First wash the area with soapy water, and then thoroughly dry it with lint-free towels and/or compressed air.
Next wipe down the area with a suitable solvent remover to eliminate any unwanted residue, such as oily films, adhesives, etc., Then sand the area with a relatively rough (40–80 grit) abrasive to provide a better surface for the filler to adhere to.
You should remove all paint within 1 or 2 inches of the damaged area to be filled. Then mix the components of the body filler on a suitable pallet. When mixed, you must work fast because the filler begins to harden as soon as the components meet. Use a putty spreader to apply the first layer of the filler to the area being worked on. Use firm pressure for the initial layer to help force the filler into the surface. Make all layers thin and don’t use any more filler than is necessary; you’ll only have to sand it off later. Drying time is about 20 minutes or so, depending upon temperature, humidity, and the specific mixture of the filler. Make sure the filler is fully hard and dry.
MUSTANG RESTORATION: BODY FILLER APPLICATION – STEP #1
Before applying body filler, metal in the damaged area needs to be worked as close to its final shape as possible. As noted in Chapter 3, there are many different of tools that can apply pressure from behind the panel and bring it close to the final shape. If it’s possible to access the panel and area from behind, there are also things like stud welders, slide hammers, picks, and so forth to pull the area into shape from the front side.
MUSTANG RESTORATION: BODY FILLER APPLICATION – STEP #2
After the metal has been worked into the final desired shape, it must be ground or sanded to abrade the surface and then cleaned to remove any dust or other residue. This helps ensure proper adhesion of the filler to the metal body panels. The paint on the adjacent metal should be “feathered” to provide a gradual transition between the areas. Make sure the area is fully dry before proceeding.
MUSTANG RESTORATION: BODY FILLER APPLICATION – STEP #3
The body filler must first be mixed to activate the hardening process. When mixed, you have limited time to apply the filler, so you need to work fast. All layers should be relatively thin, but the first layer should also be applied with firm pressure to push the filler into the surface irregularities. The filler should be applied in overlapping layers with smooth, constant strokes. Apply as little filler as possible to cover the damaged area and fill in all irregularities. Applying too much is a waste of time and material because you just have to remove it anyway.
MUSTANG RESTORATION: BODY FILLER APPLICATION – STEP #4
After allowing the filler to fully dry/harden (usually about 15 to 20 minutes, depending on conditions), it can be sanded to shape using a series of progressively finer-grit abrasive sheets. The initial sanding should remove material quickly, but not too much. In the latter stages, you should achieve the desired final shape and surface smoothness, and all visible scratches from the sanding should virtually disappear. A thin coat of special glazing/spot putty is used as a top layer before priming.
MUSTANG RESTORATION: BODY FILLER APPLICATION – STEP #5
Those with some experience and a feel for doing bodywork can often use a handheld sander to make removal of the filler easier and quicker. Take great care to not remove too much material; this equipment can easily remove too much. In that case, reapply filler and repeat the sanding step more carefully.
MUSTANG RESTORATION: BODY FILLER APPLICATION – STEP #6
After sanding is complete and the necessary amount of filler has been removed, the area often looks stratified from the removed layers of paint and filler. These different-colored layers should all be relatively thin and smooth. You shouldn’t feel any edges, scratches, or other imperfections. The surface should also be free from any waviness or other localized unevenness.
Choose a sanding block that’s suitable for the size and shape of the area being repaired and attach a 40-grit abrasive sheet. Sand the area to a rough approximation of the final shape. With care you can also use a handheld rotary sander at this stage, again using 40-grit for the disc. You are simply trying to remove material quickly; you do not want to remove too much or you’ll have a low spot, which will require more filler. Just take it slow and remove only enough filler to yield the shape you want; you want it just a bit larger than the final shape. Change the sanding sheet/disc to 80-grit to take the area down to the final shape and to remove the deepest surface scratches. You want the surface to be smooth and level, with a gradual transition to the surrounding areas (a feathered edge).
Next, change the abrasive sheet to 180-grit and lightly sand to remove visible scratches and to provide a smooth transition to the surrounding painted areas. At this point, you should be working with a hand-held sanding block only and should make sure there is minimal filler thickness at the edges of the repair area. You do not want filler on top of the surrounding paint if you can help it; you want the filler and the paint to be level.
Mix and apply a thin layer of a suitable glazing/spot putty over the entire repair area, making sure to overlap onto the adjacent painted areas. Allow this skim coat to fully dry (about 15 minutes) and then use a 180-grit sheet on the sanding block to sand the area until it is level. It is critical to keep the block flat to the surface and apply only very light pressure, to avoid removing too much material. Repeat this step, using a 320-grit sheet to remove any remaining visible scratches. Finer-grit sheets can be used as well, but there really isn’t much point beyond 400-grit or so because you want a somewhat rough surface for the primer to grip. The main objective is to remove scratches and make it smooth.
After the areas to be filled have been sanded smooth to their final shapes, a coat of primer is usually applied. This not only seals any exposed metal from the atmosphere, and prevents new surface rust, but it also helps reveal any flaws that were missed. Many defects won’t be obvious until the whole car is covered in primer; the patchwork of body filler and paint tends to mask things somewhat.
There are multiple types of primer, ranging from epoxy-based primers to those formulated for multiple layers to mask small imperfections. The type to use depends on the situation and is one of those things that an experienced shop can provide guidance on. In general, you want to minimize the total thickness of the primer and paint coats, within reason. Up to a point, a good primer can lessen the amount of sanding needed, but a thick application of primer should never be used as a way to mask poor surface preparation.
Each coat of primer should be sanded (blocked) before the next coat or any topcoat is applied. This is where the real difference is made in the final appearance. Blocking is a very time consuming process because you’re establishing the final surface/shape for painting. The cost of your paint job is almost always directly related to the amount of blocking. A daily driver will have minimal blocking, but a show car often has considerable blocking to be nearly perfect. A weekend cruiser like ours can often approach the show-car level but usually stops short of it. The difference is the show-car paint job means spending a lot of time to remove the smallest flaws, whereas the weekend-cruiser project accepts that there is a point of diminishing returns (and increasing costs). From a couple of feet away, nobody will know the difference.
For a show car that will be judged or that is to be sold at a national auction, even this level is not high enough. Only the best will do and the cost of getting there can be significantly higher than for a weekend-cruiser project, in most cases. In either case, a common approach is to apply a color coat to door jambs, hood, and deck lid openings, etc., before final blocking to help make a smoother visual transition around these areas and to prevent excessive build up of paint. While these areas are only visible on occasion, and they generally are only repaired if there was substantial damage, it is still beneficial to paint them early to facilitate masking off the main areas.
Another aspect of the blocking process is that parts that need to line up with the main body panels should be installed prior to the blocking process. This primarily applies to doors where there are character lines that run from the door panel to the quarter panel, etc. Make sure that when you are blocking everything the lines are true to each other, even across different panels and components. This is not as critical with the deck lid or the hood; they have enough adjustability built in to compensate for most misalignment.
Fenders and other exterior panels, however, should be installed prior to blocking to make sure things line up. If they need to be removed again to gain access to certain areas for paint, you can always realign things after painting. You just need to ensure you get the blocking right so it is possible for things to line up again. All of the main exterior panels, as well as smaller bolt-on parts, should ideally be painted during the same painting session to minimize color variation. Again, a shop should have a large paint booth, and this should have enough room for the car and more parts at the same time. The shop should also have a computerized paint mixing system for matching paint to any car.
A pro painter applies the primer and paint consistently yet still knows how to make minor adjustments to compensate for any problems that may arise. Even something as seemingly simple as masking the car off before applying the color coats can involve some special tricks. Pro painters can improve the appearance of the final product by more fully hiding blend lines and minimizing the potential for overspray.
Written by Frank Bohanan and Republished with Permission of CarTech Inc