Frame and Tank Molding

There are basically two types of molding that you can do on a bike. One is the elemental blending in over weld areas to make the welds seem to disappear by creating a smooth transition from one tube to the adjoining tube or to blend in welds around mounts and various tabs and brackets as seen in this snapshot captured from the web.

The other type of molding is that where you 'sculpt' portions of the frame, like around the neck area but sometimes including the tank and frame and even fender so it looks like a monolithic component.

Below are a few web images that illustrates this type of molding although it should more correctly be called sculpting.




It can get pretty radical and at a couple of times in Chopper History it was the rage and made even the examples above appear mild in comparison.

There are builders today however who practice a more refined and restrained form of molding or sculpting as exemplified by this snapshot of a tank from the Chemical Candy site.

While looking at the photos the reader might assume that sculpted or molded bikes have a ton of 'Bondo' on them but in reality most of the shaping, sculpting and molding is actually pretty thin layers of filler over major steel underlayments consisting of thin sheetmetal and bar stock.

The Process

In it's most basic form molding a bikes frame consists of adding body filler around the welded tube intersections, brackets, mounts and tab connections creating somewhat small radius 'blends' into the adjoining tube runs. How much filler one needs to accomplish this task is dependent on the quality of the tube fitment, the quality of the welds and one's personal taste. A frame having really good Tig welds usually doesn't even need any molding to look almost monolithic unless you're trying to build a showpiece and some people actually prefer to see the weld patterns under the paint and would never consider molding in the joints so it all comes down to your personal preferences and the intended use of the bike.

Before we go much further it must be understood that any type of molding material, including lead, and brazing overlays, might eventually crack and may even fall off in chucks leaving horrible looking gaps in your handiwork. How long the molding job lasts is largely dependent on how much care you take in preparation, how thick the material is applied and how hard the bike is driven. Filler applied much thicker than a quarter of an inch in depth will probably fail at some point in time so try to keep areas filled with putty as thin as possible.

The mistake most first-timers make is to attempt to create a large fillet radius using the filler material which will only lead to premature cracking. The goal is to just barely hide the roughness of the weld itself which brings us to the really sticky question of grinding down welds.

Before we get into this really deep we have to have a elementary understanding of weld beads to begin with and while I'm no expert welder I've run enough bead myself with a variety of equipment to make some relatively educated observations that are pertinent to a discussion about molding. First of all almost all 'good' weld beads will be relatively uniform in both height and width except at the points where the bead is 'broken', either at the beginning or end of a run or where the direction has to be changed. At these 'transition' points you will usually see a small 'hill' of bead piled up a little higher than the surrounding material. These hills can be ground down to match the height of the surrounding bead without much worry about weakening the weld.

When I speak of grinding I mean using a small Die-grinder mounted with a flexible oxide disc and not a rigid grinding wheel which will only produce gouges in the metal. If you use the flexible disc with care you will only be 'blending' the high spots down in a 'sanding' action as opposed to a 'grinding' action. There is a difference, but it can only be learned through experience. The key is to 'break in' the disc on some scrap metal until the edge starts to form a curl that wraps up backwards from the main disc surface creating a convex sanding 'edge'.

So remember to be safe only dress up a weld where there is an obvious 'high' spot that is conspicuous compared to the surrounding weld material and never attempt to 'smooth' down the normal irregularities of a good weld bead.

After you've dressed up the welds, assuming that you had to, it's time to prepare the tubing prior to applying the filler material. All filler requires that the base strata be clean and relatively 'rough' or have a little' tooth' so that the filler material has something to adhere to and more importantly something to anchor it in place after it has hardened. Most tubing, fresh from the factory is very smooth and has a very thin coat of milling slag called 'mill finish' and sometimes a coating of light grease on the surface. This surface finish and film has to be removed either by sanding, sand-blasting or acid etching.

Others may have other suggestions but we sandblast the entire frame so the primer has something to grab and then wash it down in lacquer thinner to remove any oils (including fingerprints). After the thinner wash we usually swab the tubes down with 'Metal-Prep', a body shop acid wash, to finely etch the surfaces. The entire frame is then washed in very hot soapy water (using Dawn Liquid detergent) to remove all traces of the acid and then thoroughly rinsed. Even though the Metal-Prep is an acid it takes longer to dissolve grease and fingerprints than it does to etch bare steel so don't rely on it to clean the tubing. In fact if you haven't washed the frame the acid will highlight places where your hands have touched the tubing. As the acid eats into the clean metal it starts to turn whitish while areas contaminated with body oil show up as rusty splotches. Using cotton gloves we move the frame back inside the shop and deliberately 'scuff up' the weld areas with a course file and/or sandpaper creating some very fine 'ridges' for the filler material to hook into.

After this cleaning process it only takes about thirty minutes for the tubing to start rusting in our climate so we usually spray the main tube runs with a etching primer stopping short of the spots where we're going to apply body filler.



There are dozens of brands of Fillers on the market but almost all of them I am familiar with are polyester resin based. In other words they're the same material as polyester fiberglass resin except pumped full of 'solids' in the form of microscopic dried resin particles, styrene, talc, calcium carbonate, glass dust, clay, chalk and in some cases metallic material. Being resins they require a catalyst to cure and harden. Buy a lot of extra tubes of the catalyst or hardener since if you're like me you'll always use more than the product directions call for. Even though epoxy resins are harder the polyesters are more flexible and are better suited to applications where there may be some flexure. Everything in these products is harmful to your health but to be quite frank I just love the smell of Bondo in the mornings. You should wear a face mask when you're sanding this stuff, which I never did, and that's probably one reason I have trouble breathing anymore.

In our neck of the woods the most popular fillers are the 3M 'Bondo' brand of course and several of the 'Evercoat' products but there are literally dozens of brands sold across the country. Bondo also comes in several 'grades'. You'll find the better stuff being handled by automotive paint supply stores but in my opinion Bondo, despite it bad rep, is every bit as good as anything else you're likely to find. The more expensive products have a slightly higher proportion of resin to filler and the filler materials are a little lighter so the salesmen will tell you it bonds better but I personally find this reasoning a little shaky.

If your planning on having your frame powder-coated you'll have to use one of the fillers that has a metallic base like 'Evercoat's Metal-2-Metal' or Alvin brand Lab-Metal since the filler has to conduct electricity in order for the powder particles to adhere to it. I personally haven't ever had a frame I considered finished enough or good enough to warrant powder-coating so if this is your plan talk over the selection of the filler with your coater before you do any work.

Every manufacturer will have some printed recommendations on how to use and store their products but from my experience I've found first of all, that this stuff needs to be stored inside your house where its not exposed to temperature extremes. It should only be applied when the shop temperature is anything over 70 degrees, preferably warmer. Don't lay it down on cold tubing. The shop may seem warm but chances are that the frame will about 10 degrees cooler than the surrounding air. Set the frame in the sun for an hour of so before you start to mold it or warm the tubing with a hair dryer or sit it in front of a small electric heater. The secret is to keep the tubing at somewhere near 70 degrees until the resin starts to cure. Getting it hotter is not necessary and can actually cause problems. A  powder coating thermometer can come in handy.

I've had the best success by mixing the filler with slightly less catalyst than the instructions call for since it slows down the cure time and I think it retains a little more flexibility by doing it this way. The key to getting good adhesion, besides cleanliness, is to completely blend and mix the resin with the catalyst. I use popsickle sticks available at most hobby or craft stores as mixing sticks. For a palette I cut up corrugated cardboard box material into 3"x5" strips or use the flexible plastic container lids like the type that comes on coffee cans. Regardless of how you mix it, the putty and hardener must be completely amalgamated since any un-catalyzed resin that gets into the your molding area will create a 'soft' spot.

The best 'applicator' I've ever found is my finger and after you've wasted a lot of time with pieces of plastic and strips of wood you'll probably agree but if you do adopt the 'fingering' approach make sure to wash your hands in lacquer thinner to remove all of the oils first. The putty stuff cleans off with some acetone and paper towels pretty easily if you don't let it fully set-up on your skin. Sissy's can use some rubber gloves.

Filler is just thick enough so that it won't flow down into cracks and crevices on its own accord so you have to 'work' it down into, and around, the weld being filled. It's better to put it down in several thinner layers than one big glob so its not necessary to try and cover all the major imperfections in the tube connection with one application. If you do need to use more than one coat let the first layer completely cure before adding another coat. Sand with 36-40 grit paper between each coat you apply. On the final coat you can rough sand it before it's 100% cured which makes the final sanding much easier.

For final sanding use the same grits that you intend to use on the rest of the frame. Pay particular attention to completely 'feathering' the edges of the filled areas back into bare metal. The correct way of creating a feathered edge is to move the sandpaper so you are sanding from areas of bare metal into the area containing the filler which is just the opposite of what feels natural to do. Minor blemishes, pockets or burst air bubbles can be gone over with a light coat of putty prior to final sanding. If you're painting the frame you can use a one part 'glazing' putty over any imperfections but if you're using a metalic filler you need to use the 'skim-coat' method.

When you're satisfied with the finish go ahead and primer the filled spots. You'll probably notice that the filler tends to absorb the primer where the surrounding steel doesn't so you'll need to do several coats of primer, sanded between coats, over the filled areas before they blend invisibly into the rest of the frame.

It should take you about two days to do a really good job of molding, sanding, spraying primer and doing some more sanding and spraying on a typical cycle frame (about 20 welds) before its finished and ready for paint. If I'm charging for the work I usually estimate 45 minutes per weld so you can see why a good cycle paint shop gets the money they do. It's all in the prep. If you do this job yourself and do it right you can save some serious money.


The following magazine articles from the 'old days' will give you the lowdown on how things were done back then. Click on each image to get a copy that's fairly readable.

This should give you plenty to ponder if you're thinking about doing something artistic to you bikes tin work.



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