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Cavity Waxing

Cavity Wax Injection 911/912

Introduction and Principle.
When applying cavity wax it needs to be kept in mind that the car’s metal needs protecting in the following areas:

Anywhere that has been welded. ‘Weld Thru’ primers burn off directly around each weld pool, and despite claims to the contrary ‘E’ Coat type finishes do not completely penetrate welded seams. Both scenarios leave the metal bare, and in the case of welded joints, often acidic compounds are formed during the welding.

Any area that is known to rust on your particular model of car: even if your example has not yet rusted there. These include ‘mystery’ rust areas that are often as a result of the internal structure trapping condensation causing rust that is hard to explain just looking at the problem externally.

All lower cavities however difficult to reach.

Equipment, Materials and Method.
Whilst cavity wax is available in aerosol form, these are best regarded as a ‘back up’ to the main waxing process. These tend to have very good penetration, however, so are excellent in areas that the main wax cannot reach.

The main waxing ideally should be carried out with a compressor and air fed gun. These can come as an accessory for a ‘Shultz’ type gun. These are cheap enough, but the lance provided tends to be rather large (8-10mm diameter) and not very long (450-600mm). However they do enable a good coverage of the car’s internals if they can be reached, as the wax can be injected at very high pressure, and so can be flung some distance. These require at least one ‘Shultz’ type canister, ideally of wax, or at a push a cleaned out underseal canister to load the wax into.

The absolute ideal is a dedicated cavity wax gun. These come with a variety of flexible and solid lances of various lengths, some of the flexible lances reaching several feet. Perfect for the job, although require filling often.

There are a number of cavity waxes sold; Waxoyl and Dinitrol are probably the best known in the U.K. although Wurth and many car paint factors will have other brands. These have not been tested, but it is probably safe to say that any brand of wax is far better than no wax at all. Waxoyl at least is available in 5L cans, and the 911/912 will use at least this, with 10L being possible, perhaps with a small reserve left over.

Both of the air-fed methods are more successful if the wax is well pre-heated. I use a single ring electric cooking hob (£14) left on it’s lowest setting (you can almost hold your hand on this hob at this setting). The lid of the wax drum is left off, and the can sat on the ‘warm’ plate. A health and safety friendly alternative is to stand the wax in very, very hot water. The water will need to be changed often though, as ideally the wax needs to have melted to an engine oil-like consistency. It’s worth shaking the can regularly to assist the warming process (lid on!).

Finally, when using high pressure injection, the wax produces a lot of vapour: it is essential to use a good quality breathing protection whilst using the wax. When the process is finished, the mask will be found to have a thin sticky layer of wax over it: much better than rust-free lungs. It may also be worth covering the external paint/glass of the car for the same reason. Whilst the wax may protect the paint, it is a little tedious to remove, and it encourages small particles to stick, risking scratching upon removal.

If the car is in use, it may be beneficial to have the underside steam cleaned, or at least pressure washed before starting work, especially should underside treatment be desired. It is also important to make sure the car is well dried before starting. This means the ideal time for this work is during the summer, but in the real world try to garage the car for a while before treatment, ideally with a de-humidifier running.

Areas for Waxing: 911/912:
This list is not exhaustive: it was arrived at by past experience of waxing, coupled with areas that have required rust repairs on a number of cars. Many of the points will require holes to be drilled, and then plugged with grommets after treatment. Remember to paint the raw edges of fresh holes. They are best drilled with a stepped drill or dedicated sheet metal drill. A conventionally ground twist drill tends to snatch, distorting the metal and leaving a less than round hole

At the base of the front panel – suspension pan/fuel tank support there is a small box section formed by the overlap of the two panels. Drill.

Aerosol wax under aluminium chassis plate and wipe off excess. Early cars: run aerosol wax in the thin rubber holding strips of front panel: these hold moisture very well, and being thin, cannot take much rusting.

The inside of the welds that join the sides of the front panel to the front of the inner wings (just a light haze on the surface, particularly the underside of the edges of the front panel). Also inside of the open box that retains the bonnet latch.

The box section formed by the suspension pan and fuel tank support (drill: probably a couple of holes to reach the ends).

Early cars: inside of each battery box. Late cars, drill under battery area and inject, also under the battery support ‘flap’ that is welded to the base of the front panel.

Front chassis legs: drill about halfway between front of leg and the start of the track rod ‘arch’. Aim lance back as far as possible to get wax right along leg. Also can be applied form inside of boot area (may need drilling).

Wax needs to be fed into the cavity behind the track rod arch: may require drilling, but this is prime rust area.

There is a cavity formed by the ‘normal’ inner wing, and an inner wing reinforcement panel at the rear of the luggage bay. On the nearside, this is the cavity that can be seen when the fuel filler pipe is removed. Later cars also have a further reinforcement ahead of the strut towers which can also be treated.

A light haze at the bottom of the ‘smugglers’ box, particularly around the spot-welded seam at the base. (This is the area below the hinged panel on the front bulkhead).

Between frame and skin of luggage bay cover (bonnet) esp. in front and rear cavities.

From luggage bay, the underside of the scuttle (panel that the windscreen wiper spindles are mounted): the whole of the underside of this panel, especially the corner areas and the underside of the spot welded screen frame join.

From the same area, the A-pillars working right up until wax vapour come out of top, inside car: be careful if headlining is in though, better to drop pressure right down and just stick to bottom 8 inches or so.

NOTE: When installing screens, worth painting wax lightly around screen aperture where covered in rubber seal.

If front wings will release rubber seal that goes between wing and scuttle, trickle thin wax down this join before replacing rubber seals, lubricated with wax.

If headlining out, then wax A-pillars downwards from inside of car. Also worth trying to wax B-pillar if possible (Targa frame if appropriate). Finally the inside of the C-pillar and VERY important, the inside of the rear screen frame: particularly the end 10 inches and as far back up the C-pillar as possible. Again watch headlining if fitted.

Under the dashboard: lance back up towards the underside as though aiming for the A-pillars again: just a haze of wax.

A surface covering in each footwell (the areas covered by the toe boards). In particular under the pedal box.

The cavity formed by the inner sill and the middle sill. You may have to drill three or four holes along here: either right at the top (flat) of the inner sill, or along the base. Some cars are already drilled here. Wax very aggressively using as higher pressure as you dare: particularly working as far forwards, and as far back as possible.

Undo one bolt from the upper door hinge and one from the lower and wax inside of the A-post.

Ahead of the A-post in the open cavity formed by the A-post and the rear of the front wing closer: particularly at the base of the area which is closed off by the leading edge of the outer sill. Also the underside of the front outer wing especially right at the top.

The outer sill cavity: you may need to drill holes along the top surface (under door step trim). Again, high pressure, aiming as far forwards as possible and as far back. Also wax back up the sill drains on the underside.

Inside of the doors: in particular all around the perimeter of the door skin, where it is crimped to the frame. Also plenty at base of frame and the top front of skin where the frame runs very close to the skin.

From under the car, lance up around the inside and outside of the jacking points, especially where the rear outer wing joins to the sill: tricky to get into, this area is critical. Also from same place, try to apply lots of wax to the front side of kidney bowl and rear of base of B-post.

Rear of inner sill area starts to curve around to form rear chassis leg (just behind kidney bowl area, underneath). Drill vertically upwards on flat underside portion. Wax forwards and back as far as possible: trying to reach rearwards up the chassis leg and into the rear of the inner sill.

All of torque tube (either form ends: ideal, or as a minimum, from the drain tube right in the middle of the car). If torsion bars out of car, vacuum tube out, as there may be rust particles that may be glued/waxed to the inner torsion bar splines. If in doubt, drop pressure down a little.

Heater flap box area of inner rear wings: just a surface haze, including heater box mounting threads.

Rear chassis legs: there may already be waxing points right at the back of these.

Cross-member between rear damper turrets: may have to drill, but particularly try to get wax to each end where it joins the chassis legs.

Rear cross-member: may be worth drilling from underside.

Between frame and skin of engine lid cover: particularly back down onto the top of the rearmost seam (nearest number plate when down.

Rubber seal pressing in top of rear valance (as per early front panel).

In addition all of the underside can have a surface covering. The degree to which this is applied is down to cosmetics: road grime tends to stick to cavity wax applied externally, so many choose not to apply at all. My personal choice is to apply it everywhere except the very visible parts of the middle of the wheel arches; i.e. if you can’t see it with the car on the ground, wax it. There are dedicated underbody waxes, but they seem to be coloured, rather than clear: perhaps worth doing your own investigation.

The critical areas of the underside are as follows:
Headlight/indicator areas of front wings.
Fuel filler area.
Whole of rear front wing/inner wing area.
Suspension pan and tank underside.
Sill – floor seam all around.
Whole of back or B post, kidney bowl, rear window underside, oil tank area (and N/S equivalent), rear light area.
All wing wheel arch returns.
Rear quarter bumpers.

Whilst the cavity wax will last for many years, any underside waxing should be steam cleaned and reapplied as required from time-to time: much will depend on how often, where and when (season) the car is used, but inspections every other year would be a good starting point. As always, though, prevention is much, much quicker and cheaper than cure.

The above process should take little more than a day to perform, with material costs of around £40-60 maximum. Or about half the price of a genuine sill ….

Barry Carter
09 January 2007

(Barry is a keen supporter of DDK, as are we!)

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