Firewall Cleaning, Repair and Painting: Part 2
Now that the firewall has been cleaned, and all of the leftover bonding compound has been sanded away, my focus switched to repairing several areas damaged fiberglass. Unfortunately, during separation of the front clip from the firewall, and removal of the firewall from the body, the firewall fiberglass sustained damage in several locations. Prior to painting and reinstallation these areas of damage need to be repaired.
In total there were about a half dozen spots that needed attention. I’ll show the fiberglass repairs that were completed to a couple of these locations as an example. The first area requiring repair was located in the passenger foot well. The damage included a crack in the firewall as well as damage to the surface of the fiberglass. As best I can tell, this damage occurred when the firewall was separated from the rest of the body. During removal the fiberglass cracked and the top layer of fiberglass was pulled away leaving a very rough, uneven surface. The two photos below show the damage.
To start it’s important to understand that, when repairing cracked fiberglass, you need to create a repair that extends the full thickness of the fiberglass panel. Creating a partial depth repair will simply result in a weak spot that will ultimately crack again in the future.
To begin the repair a die grinder with an 80 grit sanding disk was used to sand down the repair area. In the immediate area of the crack the full thickness of the fiberglass was sanded away with the surrounding area feathered into the remaining panel. The areas showing surface damage were also sanded down to remove stray fibers and to provide a uniform surface to accept the repair areas.
After sanding the repair area it was thoroughly cleaned with lacquer thinner. Next, a piece of cardboard backing was taped into place behind the repair to provide support for the wet fiberglass.
With the repair area cleaned and ready to go, a few small pieces of fiberglass mat were torn off a larger sheet. It’s much easier to do this beforehand – the resin gets your fingers quite sticky which makes tearing away small pieces of fiberglass difficult. It’s important to note here that you want to use fiberglass mat (pictured below) rather than fiberglass cloth. Fiberglass mat has a randomly orientated strands whereas cloth is a woven fabric.
With several small pieces of fiberglass mat ready to be applied a small batch of fiberglass resin was mixed in a glass jar. The resin hardens in about 10 to 20 minutes, so be sure to mix small batches. For these repairs, I used Evercoat fiberglass mat and resin which I’ve heard good things about.
The fiberglass resin is applied using a cheap paintbrush – the cheapest you can find. Cleaning the brushes is pretty tough and not worth the effort so don’t buy anything expensive. Start by applying a thin coating of resin to the area of fiberglass which will receive the repair.
Next, the first piece of fiberglass mat was applied and wetted down with the brush. Once the first layer becomes transparent the next layer of mat is applied and wetted down. This process is repeated until the desired repair thickness is developed. The goal here is to develop a fairly ‘dry’ repair with no air bubbles. The fiberglass mat is what makes a strong repair so you want to avoid using too much resin. Once the repair was built up to the desired thickness the excess resin was removed from the brush and the repair area dabbed with the brush to remove excess resin.
After allowing the repair area to harden overnight (several hours probably would have sufficed), the repair material overhanging the edge of the firewall was cut off using a Dremel. Once the trimming was complete, the entire repair area was sanded smooth using a DA sander and a die grinder – just be careful not to get carried away. If you remove too much material or you’ll be repeating the process all over again.
With the first repair complete, I moved onto fixing a second area of fiberglass damage in the drivers foot well. This area was repaired using the same procedure as above.
Fast forwarding a few repairs, it was time to prepare the firewall surface for paint. There was quite a bit of grime, dirt, and excess bonding compound on the surface of the firewall that I wanted removed before painting. After a good cleaning and degreasing, a die grinder was used with an 80 grit scotch-brite pad to provide a smooth uniform surface for painting.
I should note before going any further that during the cleaning and painting process I ended up removing and painting over the original factory writing on the firewal. There are some purists out there that go through great pains to preserve or recreate these factory markings. The attention to these small details are what create truly remarkable, original, cars that bring in high prices at auction. Ultimately, the condition of the car should be the determining factor on whether this is the right path for your project. A relatively clean original numbers matching car, with all of the markings from the factory (even in wax pencil) still visible is probably a good candidate. If this is an option, care should be taken to document as much of the original condition of the car as possible so that it can be recreated later if needed. Although this is an option for some restorations, for my non-numbers matching car I didn’t feel it necessary.
After several hours of cleaning the firewall was dry fit to the body to ensure the repairs that were completed didn’t adversely affect the firewall fit up. It would be a pity to have to modify a freshly painted firewall if things didn’t line up, so this is a good time to double check everything. Thankfully, both sections lined up very well and no modifications were required.
After another thorough soap and water cleaning, followed by wiping the entire surface down with a pre-paint cleaner, it was time to apply an epoxy sealer/primer to the bare fiberglass. For this task some leftover PPG DP40 epoxy primer left over from my windshield frame repairs was used. Using an epoxy primer is particularly good for this application because it holds up well against solvents such as gas and oil (very common in engine compartments). It will also provide an excellent bonding surface for any lightweight filler that needs to be applied in order to provide an even, uniform surface. In fact, many folks will say that you shouldn’t apply body filler directly to the fiberglass at all. Rather, the fiberglass should be painted with an epoxy primer first and the body filler applied over that.
With the primer applied the firewall looked pretty good, but there are still a few minor cosmetic issues that require attention. In particular, the grain of the fiberglass is quite visible in some areas. In other locations there are divots and minor gouges in the firewall leftover from the disassembly process. These will be smoothed over with body filler followed by the application of a high build sanding sealer and final top coat. After all this work the firewall should come out looking great! Plus, this provides an excellent opportunity to hone your skills before starting on the actual body work that most folks will see!
Stay tuned for the next installment and the finished firewall!