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How to Smooth Your Firewall

6/2/2020
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It’s all in the details…

Ever since automobiles became an alternative mode of transportation, car enthusiasts have been upgrading engine and body parts and sharing their ideas with other gearheads. It just seems to be a basic, ingrained human behavior. We’re compelled to be different and modify our cars because, well, we just have too.

One instance of this is when a good friend of mine asked me to help him and his wife on their S-10 project, he has been building the truck as a 1/8 mile grudge race truck that is also to be streetable. Since it is also going to be driven on the street and to the local show and shines he also wanted under the hood to look stock and cleaned up nicely rather than an aluminum firewall like most strictly drag race-prepped vehicles have.

We began by disassembling the front of the vehicle. He already had the motor out and at the machine shop for a new build so we had full access to the firewall. We removed everything from the firewall so we could start with a blank canvas. By removing the heater box, steering column, booster, and all other miscellaneous items that use the firewall for support, it would make quick work of the other steps to come.

After everything was removed and labeled, we decided which holes in the firewall were no longer needed. This would pretty much be all of them minus the steering column and brake booster mount holes. Since this would be a drag vehicle the track rules state that the firewall cannot be cut or removed, there was only a few smaller holes so we opted to just fill the smaller holes and grind it smooth, rather than cut out the entire firewall and replace it with a piece of sheet metal which can be quicker in some instances. We then created templates out of poster board from the craft store to fit in each hole. We cut them out and transferred them to a similar gauge sheet metal thickness as the firewall.

Once all the pieces were cut to fit and edges were cleaned up. The holes on the firewall were also ground down to bare metal to prep for welding. Small magnets were placed around the filler piece on the inside of the truck, so we were able to keep them in place on the firewall while we welded on the outside.

Once everything was welded and ground smooth, we ran over the entire firewall with 320 grit on a D/A sander and used smaller pieces of 320 to scuff areas the D/A could not reach. We then started the bodywork and filler work prep by removing any dirt or dust with solvent wax and grease remover. We applied one quick coat over the entire firewall panel.

The filler we used was from our local body shop supply store which uses a small tube of hardener. Once it was set up and able to be sanded, we applied a guide coat. This will allow us to see our low spots once we start sanding with 120 grit on a smooth block. After everything was sanded, we noticed there were a few low spots which we expected. We went back over and applied a glaze coat to the low lying areas. A glaze coat is similar to filler except it is easier to sand once hardened and is only intended for smaller areas.

Once everything was sanded and smooth to our satisfaction, we went back over the entire area with 220 grit sandpaper by hand. We then wiped everything down once more with a wax and grease remover to prevent fisheyes and other imperfections in our primer coat. After masking off the entire truck to prevent overspray and wiping off once more just before spraying, we were ready to apply some primer.

Using a Titan Tool 2.3 MM Tip Spray Gun we were able to layout a nice smooth base coat of primer. Apply 2-3 heavy coats of primer to fill in any sand scratches that might be leftover from the 120 grit sand down earlier in the week. The primer should have taken care of most of those for you. We let it sit overnight and came back the next day to smooth out the primer by wet sanding the area with 600 grit wet sandpaper and a bucket of water. After re-masking the truck in preparation of some color the following day, we were then ready to shoot some paint knowing we had a good smooth foundation.

By going with a “Hot Rod Black”, which is a matte black with a slight gloss for the firewall, it will be subtle enough to not draw your attention away from all the details of the engine, yet look different from a factory S-10. One downside to a flat or matte finish, unlike gloss, is that if you do have any issues such as “sags” or “runs” in the paint it cannot be wet sanded or buffed out. This can happen where you have too much material at once in one area. With a few test spray panels using the same paint gun as earlier, we were able to achieve a perfect finish that was ready for assembly.

We do different things to our vehicles typically for a certain reason. Whether it be to go faster, look cooler, or just to be different. It is not always about getting that new intake or headers installed or the new cowl hood painted for your hot rod. It’s the hours spent wrenching in the garage or driveway and meeting up with your buddies at Sonic or Culvers on the weekends and sharing your new tips that makes it all worthwhile.

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