'55 car rust repair Part 10: Filler applications

[Daves55Sedan]

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This section deals with application of rust prevention treatments, primers, fiberglass epoxy resin, different caulks and use of bondo. Final finish painting is covered in the appendix.
First: All patch panels were painted with a cold galvanizing compound on the backside and a cheap aerosol spray enamel overcoat PRIOR to being welded in place (except for the replacement left rear quarter panel which was painted on the backside with the Dupont "Chroma-base" metallic blue base enamel). I found that after welding, the galvanizing primer and enamel on the backside suffered little damage by the heat. The backside of the outer rocker panel patches got a second coat of enamel after the welding, because I was able to poke the paint spray thru the large void on the backside where the inner rocker was cutout. (typical both sides of the car). The only place where there will possibly be rust formation is on the backside of the inner rocker panels where they are spot welded at the very bottom to the outer rocker bottom flange. The weld seams of the tailpan patch pieces are unaccessible for painting another coat on the backside after the welding was done due to the tight space at the back of the trunk floor. Time will tell if rust occurs at these weld seams from underneath.
Every new metal patch small and large was fashioned to fit in a cutout hole in the car body so that the new piece fitted flush with the existing remaining body panel. I do not like overlapping a patch on top of a cutout, because you have to make up the difference in the metal thickness with bondo all around the patch. Theoretically it ends up looking like a "bump-out" in the area that was patched. Its easier to overlap-patch than making patches flush, but there's more bondo work to overlap patching and it still doesn't look right. If this project had been done at a body repair shop, large rustholes would have been overlap- patched and small holes would have had the rust hole beaten inward with a hammer and fiberglass patched over the rusty hole. That is not quality work IMO.
I did NOT do continuous welding. In most cases, weld seams were filled with beads of weld 3/8-1/2 inch long, leaving a 1/4 inch or less space along the seam, then repeated that pattern for the length of the seam. Weld seams of all the large metal patches were fashioned with a step-flange using the pneumatic press tool, or instead had strips of metal welded on the backside so that when the panel was fitted in place on the car, the panel would be flush with the surrounding existing metal panel. Another advantage to this is that the flange prevents "burn-thru" during welding. The gaps between the weld beads would need preliminary treatment.
Second: Preliminary treatments of the weld seams began with grinding the weld beads flush with the panel surface. The grinding was mainly done using a electric angle grinder with a coarse stone grinding disc. To avoid gouging into the surface of the metal panels, I switched to using the Dremel rotary tool outfitted with a 1/8 inch thick grinding disc. The Dremel tool was essential in smoothing down the corner welds at the tailpan. In the interest of preventing rust at the seams, I did not apply any bondo directly into the weld seams. Portions of vertically oriented seams that were still exposed (without weld beads) were filled with JB-Weld or with fibreglass epoxy resin (held in place until dry with blue masking tape). I did not use the yellow masking tape that is really sticky at the backside for fear that when the masking tape was peeled off, it might rip out my epoxy filler along with it. I used the blue masking tape (similar to frog tape) popular with painters. This tape is less sticky and peels off with ease. Using this tape was successful in holding the resin and did not peel it off when the tape was removed. All horizontally oriented weld seams were filled with fibreglass epoxy resin as there was no need for tape on these. Any small metal patches that had no backing and were held in place with magnets for welding leaving a few small gaps in the weld seam were filled with JB-Weld. I consider JB-Weld and fiberglass epoxy resin as basically non-porous, thus theoretically would not absorb moisture after they are dry. That is why I used these as preliminary seam treatments as opposed to bondo which does absorb moisture and could lead to premature rust at the weld seams.
I also used JB-Weld to patch the weld seams of the new metal patches at the front of the rear wheelwells. The reason for this was because I believed that eventually rocks would pummel these areas so I wanted something much harder and more durable than a caulk or bondo. I believe using bondo here would have been a huge mistake. In fact, there was NO bondo work at all done anywhere on the undercarriage.
Third: Bondo was used where needed to smooth out where needed over the preliminary treatment. I had saved cardboard food boxes with the shiny side (like cupcake snack boxes) for many months and cut them to use as bondo mixing/serving surfaces. You can only use a sheet from this cardboard once. If you need to mix another batch, you need a new piece of cardboard. Bondo sets up very quickly so you can't mix a really big batch and you still need to work fast to use all that you mixed before it sets up.
The plastic bondo trowels I used have been here since the 1970's and are very used. I have one complete set of all the available widths. They can be kept in good enough condition to be used over-and-over again if they are properly cleaned and cared for. Instead of trying to peel off dried bondo residue from the trowels after applying a batch, I would keep a soft cloth soaked in gasoline and use it to wipe away the residue. I kept the same old coffee can with snap on-lid filled about a quarter full with gas and the cloth inside all through the finishing project for this. Also, I kept little pieces of cardboard cut from snack cake boxes to use as cleaning trowels to remove residue from the plastic bondo trowels before bondo had hardened so that the gas-soaked cloth would not get gummed up with lots of bondo. Any bondo that I scraped off the trowels would be quickly applied to an area of imperfection or a weld seam that needed more bondo before it dried, thus very little bondo that was mixed would be wasted. It is not recommend to ever scrape hardened bondo off a plastic trowel using a putty knife as this can easily scratch and ruin the surface of the plastic trowel.
For the whole project, I used two tubes of JB-Weld and about 4 ounces of fiberglass epoxy resin. I started out with a quart of bondo for the whole project. By the time all the bondo work was done I still had about a quarter of the can leftover. I am not very diligent with bondo. I probably filed/sanded off at least half the bondo I applied to the weld seams. It was very time consuming and somewhat wasteful, BUT, I dreadfully feared that if I had taken the car to a body shop to get this work done, it would be VERY expensive, would not be done right, AND would be rusted through again in a few more years because they would cut corners and not take all the steps needed in rust prevention.
Fourth: After all filler work was dry, I used a large bastard flat file to begin with smoothing the bondo, then progressed down to a fine file. I also have a bastard and fine files which are flat on one side and curved on the opposite side that proved to be very helpful. For very long weld seams, I used an adjustable rasp to grind the surface of the bondo down smooth. It turned out that I did not use my sanding discs (for use in a power drill), nor did I break out my orbital sander at all. After filing/rasping was finished, I used sandpaper stapled to the sides of a 1 X 4 piece of lumber for the rocker panel patches and for the right rear quarter patch. I started with 320 grit just to get an even flatness, then changed to 400 grit for more smoothness. Most folks would go to an even finer grade of sandpaper to finish, but I am not building a trailer queen here. The only purpose is to repair holes and to hopefully prevent new rust from occurring. I dry-sanded on bare bondo and kept the sandpaper clean with a small, fine wire brush. For the whole project, I picked out one new 320grit and two 400 grit pieces of sandpaper. When all the sanding was finished, these pieces of sandpaper are still useable. After the first coat of primer, I wet sanded with the 400 grit paper. Waiting till nightfall, I used a trouble-shooting light to inspect all the primered areas to see if there were any valleys, or flaws (which could only be seen by the light casting a shadow at the flaw). Any discovered flaws were encircled with a black magic marker. Then, I poured some old Ditzler grey metal primer in an empty tuna can and used a small bondo spreader to trowel primer out of that can and spread the primer over the flawed area which had been previously marked. Each area that required more primer as a filler was sanded smooth and again inspected. Some flawed areas required a little more bondo spread over the area. I did not take great pains to try to smooth out tiny flaws in the bondo that would most likely not be seen unless you had your face a few inches away, since this is not by any means a showcar. After working out all the major flaws, I primered over the bondo and any bare metal. I wet-sanded and buffed out the final coat of primer with fine steel wool by hand for a little extra smoothness. The primer that I used was a cheap grey automotive aerosol spray from one of the local chain auto parts stores. For the whole project, I used two full cans and part of a third can.
Fifth; Caulking and seam sealers; I used "Black-Jack" roofing cement to caulk all welding seams under the car along the inner rocker panel patches. I also used it to caulk weld seams where the wheelwell patches connected to the lip of the fenderwell (at the front, opposite where the gravelguard goes). I used 3M "Fast-N-Firm" caulk to seal the weld seams along the underside of the wheelwells (where the fender outlet meets the wheelwell flange) and also the very bottom seams of both rear quarter panels where the inner panel connects with the outer panel. After allowing to dry a few days, I went back and sprayed two coats of grey primer over the caulked areas using aerosol cans.
Weld seams having the step-flanged edges or the tack strips on the backside were sealed on the backside after welding. For long seams, I used some of the 3M Fast-N-Firm caulk that was leftover from the car rain gutter repair project (covered in an earlier unrelated post). It is the hope that the caulk will prevent any condensate caused by "sweating" from running down inside the fender and laying in the flange at the seam where it would begin to rust and eventually cause a hole to form. Any areas that I could not access on the backside with the caulking gun, I treated with "Black-Jack" roofing cement. It happened to be my luck that I had a partially used caulk tube of this stuff. I cut the caulk tube open and trowelled the material out of the tube with a narrow putty knife then applied the caulk to the area that I wanted to protect using the putty knife. It turned out that all the areas where I needed to use this caulk were finished just as I ran out of the last glob in the tube. Nothing short of miraculous there if you ask me. All weld seams and joints between body panels that were caulked with "Black-Jack" or 3M "Fast-N-Firm were primered over so that the overcoat would stick. I had to spray primer the areas caulked with "Black-Jack" because trying to brush paint these would cause the primer to mix in with the cement (and gum up the brush). I used my old can of Ditzler Kondar primer to paint over all the "Fast-N-Firm" seams using a wide modelling brush.
This is the end of the rust repair ten part series, but there is an appendix coming soon that only covers the trials and tribulations of the final enamel overcoat painting.