If you’ve been following our Forgotten Fastback build, you’ve seen all the sheet metal we’ve replaced on the car and last time out we assembled this rotisserie and we put the car up in the air so we’d have access to the underside of it. Now that the sheet metal is finished, our next step is going to be seam seal all the joints and undercoat the body.

We’re going to start with our caulk gun with this joint and seam sealer to the larger areas, particularly around the rocker area. Once those are all filled in we will use the Eastwood seam sealer for our smaller areas around the floor, floor supports and torque boxes. Once we’re finished with that we’re going to undercoat the entire thing using the Eastwood rubberized undercoat.

From the factory our Mustang would have had seam sealer all along this rocker edge here, around the torque boxes, in to the edge of the rear floors upfront. Since we added torque boxes in the front of our Mustang for increased support, we’re actually going to seam seal this whole area. We use a larger seam sealer for all of this and use a brushable for the smaller sections here. You can also seam seal anywhere you have metal overlapping each other. It wasn’t done that way from the factory, but if you’re building the car and you’re going to drive it, it’s not a bad idea.

I’m going to start with the edge right behind the torque box; lay a nice thin bead down. You’ll want to go a little thicker depending on the gap you had between your rocker and the floor. Let me get the inside of the frame here behind the torque box. Now we’re going to do this section here around our floor support and our torque box. Before I can seam and seal our firewall to our floor pan, I got to make sure it’s a nice tight fit in the transmission tunnel. We’re good all through here. There’s a little bit of a gap up here, so we’re going to hammer it flat before we seal it.

Now we’re going to move on to our brushable seam sealer; get the areas in here, on our floor supports and everywhere we have overlapping metal that wasn’t thick enough to require the joint sealer. You apply this with your brush and just kind of spread it out over the area. Make sure you get up in to the groove between the two metals. Once you’ve done your seam sealing you want to wait at least 30 minutes. We suggest closer to an hour before you’re ready to undercoat.

Now we’re ready to spray our Eastwood undercoating. You want to put tape over anything you don’t want to get undercoating on. In our case, we can use a piece of cardboard just to keep it off the body of our car. Since our car hasn’t been painted, there’s nothing else on here that we’re worried about painting. We can just spray undercoating across the whole bottom of the car. You’ll want to make sure you’re wearing a mask and eye protection when you’re spraying your car, in a well-ventilated area as possible.

Figure out about four to five cans to get a nice heavy coat on the underside of your Mustang. Now that our chassis undercoated, we’re ready to move on to the next phase of our Forgotten Fastback.

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