Insuring a waterproof Ute cabin is of utmost importance.
One consequence of cutting the Beetle in half is that a new barrier between the inside and outside is created along the truck bed front wall. The chassis under the truck bed is filled with holes and inaccessible cavities so keeping water out is a critical matter. See also sunroof drains.
Of primary concern is the long seam of the front truck bed wall that communicates directly with the rear interior floor/carpet as well as the large side channel opening by the rocker panels.
The side channel slopes down towards the cabin and, if exposed to the weather, would become a catchment for water from the front third of the truck bed undercarriage area. This would result in water draining onto the carpet near the seat belt floor anchors.
Mitigation of this issue is an absolute necessity.
VW engineers never considered the need for significant luggage area water drainage. There are two small front drains holes on the left and right — one over the gas tank and the other by the exhaust heat shield — but these would not help drain water away from the stepside cavity near the rocker panels due to the topography of the chassis floor.
The significant vulnerability, aside from leaks along the front wall seam, is that water would flow through the gaps near the arch support and begin to fill the stepside cavity like a bathtub!
Treating the seams around the truck bed frame has several advantages. First, water flow will be directed away from the wall surfaces making it more likely that the drain holes collect as much moisture as possible.
Second, sealing keeps road grit out of the floor edge seam and facilitates a bed clean up when necessary.
And finally, sealing tends to minimize rattles caused by metal-to-metal contact.
We elected to seal the seams after applying a first coat of Raptor bed liner. A second coat will completely cover the sealant and provide further protection. If the bed walls are left untreated (that is, raw or polished aluminum) the esthetics of seam sealing becomes more complicated; a clear sealant may be more applicable.
Another mitigation would be to add a “wall dam” that extends down from the front part of each side wall under the truck floor.
The “dam” would create a “wet” middle compartment, leaving the right and left sides by the rocker panel relatively “dry”.
An further benefit is that the dam would create a water-resistant area for mounting the rear speakers through the front truck bed wall.
A second smaller dam adds protection against water flowing down the side channel slope. Although this area should be “dry” due to the protection of the larger wall dam, this is an insurance policy against any water that does make it into the side channel. When the cavity is filled with foam, the result should be waterproof.
Side cavity treatment
The large side cavity that drains directly into the cabin clearly needs to be addressed. One method is to fill it with 3M foam and then rivet on a top cover. This will prevent water from oozing down inside in the cabin.
Sealing the edges along the truck bed front wall and installing the wall dam are still necessary.
After all the chassis holes are covered (except for the three drain holes — one in the spare tire hold and two towards the front), the entire under-bed surface can be treated with a rubberized undercoating.
All the mitigation steps described above still leave the stepside cavity vulnerable to holding water for extended periods of time even if this water doesn’t leak into the passenger compartment.
The cavity can be completely filled with waterproof foam but this interferes with the speaker enclosures and “buries” any wiring in foam forever.
An alternative approach is to install a small drain tube at the bottom of the cavity; fortunately there is a chassis plug directly underneath that can serve as a convenient outlet.
Fashioning a suitable drain tube is relatively simple. A small washer acts as a mount with a rectangular cutout on the tube that faces the low point of the cavity. This eliminates any “lip” that might impede the flow of water. The outlet just goes through a small hole in the plug.
Level chassis floor depressions with fiberglass resin so that water will flow towards the drain and not puddle elsewhere.
When leveled properly, water flow will be directed to the small drain hole (blue circle) and exit the cavity. Remaining small puddles should evaporate since there is still some air flow through the wheel arch and truck bed.
Materials and procedure
A combination of materials is recommended since no single sealant is likely to handle all aspects of the required mitigation.
Front wall seam
Our choice for the highly vulnerable front wall seam is 3M 8308 Heavy-Bodies Seam Sealer that is dispensed in a dual 200ml cartridge. This sealant is relatively expensive, requires a special application tool (3M 8571), but is specifically designed for exterior seams and joints. After an etching primer is applied for insure good adhesion, 8308 cures quickly and dries into a non-sag hard surface.
The various deep pockets of the chassis are best filled with 3M 8463 Flexible Foam (another dual 200ml cartridge) that is tricky to apply. The material, a thick paste, plops into the cavity and then starts to expand; care must be taken not to apply too much or it overflows and then must be cut back. Cutting the foam voids its waterproof skin so it must be coated to make it waterproof.
The inside of the cabin wall should be taped so that foam flow isn’t unrestricted. The material will bubble through the cavity and ooze out along the edges of the channel dam. Foam will stop most of the water, but small cracks may still exist; the only safe method is then to completely seal the outside exposed surface.
Given the uneven surface, we used 3M 8307, a self-leveling sealant in a 200ml dual cartridge, to completely cover the foam in the cavity. This creates a solid join between the front wall and the chassis floor.
There remains a small area around the harness pass-through that could leak. As a precaution, 3M gasket seal is applied over the foam sheet loom protector.
See Panel assembly for a discussion of how to fabricate a profile to fit the uneven chassis floor. Seal along the dam edges with 3M 8463.
The truck bed and substructure seams, including the B-pillar gusset plates, were treated with 3M 8300 Ultrapro Autobody Sealant, because this product adheres to painted or raw metal and remains flexible enough to not crack under road conditions.
When the mitigation steps are complete, the moment of truth arrives … is the cabin waterproof? All water testing should be done before any fiberglass body shells are installed. Washing down the truck bed with a hose is a good way to check. There should be five draining points — the two front drains, the rear tire hold drain, and the two stepside tubes — that direct nearly all the water. Some water will likely remain in the hollows of the chassis floor and evaporation is the only way this moisture dissipates. Obviously, there should be NO water anywhere inside the cabin!