Undersealing A MK4

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Undersealing A MK4

Postby drumtochty » Tue Oct 27, 2015 1:10 am

The following is a guide to topping up the rust proofing of a new or nearly new Mazda MX5 MK4 that has not spent a winter in the UK. This work was done by members of the MX5 Owners Club (Grampian) in the North East of Scotland and includes lesson learned after doing two cars in October 2015.

Mazda UK have advised a Mk4 owner that they stonechip certain areas underneath the car but do not fully underseal the car.

To date the team have not removed the door cards to coat the inside of the door panels nor for that matter did they lift any sill kick plates / trim at that area. These will be included in future revisions if relevent.

While it is possible to do the job with a car on axle stands, one of the team had a two post lift and the work was completed on that equipment

Ultimate Results

As Mazda have installed various equipment and covers over non stone chipped areas underneath the car that cannot now be coated during this process, it is possible over the long term that these areas may well corrode but short of removing major components and heat shields nothing can be done by the owner to coat these hidden areas that could easily have been completed at the Mazda factory.

As always in these circumstances it is the persons working on the car who is responsible for their safety, this article in no way advises owners what is safe to do at home with limited facilities, especially if the owner is doing the job on their own.

We have only considered Dinitrol and Bilt Hamber products.

For work in the engine bay and boot where the job is seen we would suggest clear products either.
2 off 750ml aerosols of Bilt Hamber UC clear wax or
3 off 500ml aerosols of Dinitrol 4010 Corroheat clear high temperature wax

Reduce by 1 can in either case if you do not do the boot area.

Increase by 1 can in either case if you intended to do the inside of both doors.

For the exposed underside remember you are only doing the area missed by Mazda the recommendation is
If you want to see the existing paint colour underneath.

2 off 750ml aerosols of Bilt Hamber UC clear wax or
3 off 500ml aerosols of Dinitrol HP underbody wax or

If a mild browning of the wax is acceptable but not jet black
2 off 750ml aerosols of Bilt Hamber UB underbody wax or

If a black finish is required
3 off 500ml aerosols of Dinitrol 4941 underbody wax or
3 off 500ml aerosols of Dinitrol Black 445 stone chip.

Bilt Hamber advised that all the available blackeners when the UB wax was formulated had other drawbacks with repect to holding dampness but they will look at the question again.

For the internal body cavities, inside the chassis rails, inside the subframes and any area protected by wheel arch liners the recommendation is
4 off 750ml aerosols of Bilt Hamber Dynax S50 or
6 off 500ml aerosols of Dinitrol 3125 Cavity Wax

These are used with supplied 600 mm long wands with 360 degree nozzles for cavities and 100 mm long nozzles for most other work.

You also need various newspapers or polythene sheets to protect the floor and some degreaser and hot water to clean dirty areas.


Some way of getting the car in the air.
A few plain screwdriver to remove the plastic cover clips and none were damaged.
A small posi drive screwdriver to remove one clip on each of the back wheel arch liners.

The following sockets, 8mm, 10mm, 12 mm, 14mm, 21mm
Bucket and cloth
Rags or paper towels
Gloves, overalls, protective footwear, close fitting hat.
Some people use disposable overalls
Two under car stoppers to close the hole that is used by Mazda during sea transport, if Mazda did not fit them on delivery get them from your dealer for free, if they have not been fitted. They know all about them but rarely fit them during the PDI.


Preparation for Undersealing the Car.

Lift the car in the air in a safe manner and support it, with a 21mm socket on a whizzy gun. if you have one, get the wheels off, you also need the anti theft wheel key. The club member who has the garage has a torque wrench set to 110 NM that they always tighten wheel studs with prior to driving any car after maintenance.


Cover Removal Listing

Rear section of front wheel arch protector.
Front section of front wheel arch protector.
Both of these on the other side.
The front black under shield.
The aluminium sump under shield.
The front section of the rear wheel arch protector.
The rear section of the rear wheel arch protector.
Both of these on the other side.
The nearside rear cover behind the rear firewall.

Removal of covers

The front wheel arch protectors are PVC and come off in two pieces. If looking at the nearside wheel, one section is from the back at 3 O’clock, towards the front of the front wing at say 11:00 O’clock and is held on by about 15 plastic clips, you need a small screwdriver to remove the clips. The front part of the wheel arch protector and the attached front wing under cover is held on by about 5 plastic clips and 4 off 8mm AF bolts. Take a picture of where the wheel arch protector covers go behind the front wheel arch return. We got it wrong the first time we replaced it.

This is a front section of the front wheel arch protector and a view of the front engine under cover.


See it placed on a wheel


The rear wheel, front section of the wheelarch protector are not PVC like the front of the car but are a fabric which we think may hold dampness! They are held on by a few plastic clips, some say 14mm AF plastic nuts that are only finger tight, two 8mm metal bolts and one clip that is fixed with a small posi drive screw that was the most difficult to remove without breaking the clip. It needed turned and pulled with a set of plyers at the same time.


This shows the plastic bolts on the rear liner and one of the 8mm bolts.

Watch when taking off the rear plastic cover for the rear arch, slip it off so as not to break these clips.


This front centre under cover needs removed and is held on by 8mm and 10mm bolts and plastic clips.


The sump cover needs removed that needs both 10mm and 12mm sockets to remove the fixings.


Finally you need to remove a black plastic cover near the nearside rear wheel. It is held on by 4 off 12mm bolts.

If Mazda fitted the lower cavity bungs they need taken out with a plain screwdriver.

Allow a few hours to get to this point, if there are two of you about an hour.

Now you need some degreaser and a cloth to go over all the areas underneath the car that you intend to apply wax to as you do not want to apply it on to dirt or dust or grease.
Again a good hour or two depending how many people are doing the job.

Remember, this article is about a new or nearly new car, on an older car, you would have a day or so removing rust and applying rust convertor and priming the repaired areas but not in this case.

Application of Rustproofing

It is now a case of putting the long flexi nozzle on the cavity wax cans fully inserting in the various areas and slowly removing while spraying and trying to turn the nozzle at the same time.
The areas to be done are:-

The holes half way along the sills and you go vertical, left and right in there so three applications. This uncoated part of this access hole also needs covered by whatever product you are using on the underside.


The small chassis rails under the car four or five entry points on each side.


The front chassis rails from the front bulkhead to the front bumper two places to go in there at least. Note where the back of the front chassis rail has been painted with Bilt Hamber UB, looks dirty brown rather than undersealed. In reality a good coating but appearance says not undersealed.
Lots of area to do as you see.

Inside the front subframe.


A number of cavities around the front wheel arches and around the front of the floor and the front bulkhead.


Under the front wings and behind the vertical part of the front wings note again the remark about missing areas when you apply the goo.


This is the front wing area that is protocted by wheel arch liners but on a wet day the water gets in as you see, now what is the score with a daily driver in the winter.


Another under wing cavity.


With a good coating of Dynax S50, notice what I said about checking for missed areas in the QC sectiom

The area inside the rear wheel arches including inside the return where the wheel arch liners sit.

Underbody Wax or Stone Chip Your Choice

Outside the front chassis rails.


On the front sub frames.

On the rear sub frame


The area where the handbrake cables come out the cabin is not stonechipped.

On all bolts and exposed threads under there.


On the various pick up points between subframes, etc. and the body where no underseal has been applied.

The area behind the rear bumper but not the bumper bar.

The area around and under the heatshield above the rear exhaust box.

This has been done

The rear bulk heads as you can see getting done.


The area around the rear cabin drains.


The area under the plastic sill protectors.

Various areas on the chassis rails that have been missed during manufacture.

The area behind the front bumper or at the very front of the front chassis rails.


Use the clear wax to cover any painted bodywork in the engine bay, the exposed areas under the bonnet and in the boot.

Quality Control of Work

One person goes round with a torch and the other follows with a can and treats the missed areas.

It is very difficult to get it right first time and there are always areas missed.

Remove gloves and replace with clean gloves.
Put empty tins in rubbish.
Replace Covers.
Replace the rubber bungs at the holes towards the back of the sills.
Most of this work is just the reverse of taking off points to remember.
The rear sump cover holes go under the rear plastic cover.
Check the front wheel arch protector / front wheel arch return is to the picture you have taken.
Torque all the wheel studs.
Overalls off and get cleaned up.
Last edited by drumtochty on Mon Aug 07, 2017 10:51 pm, edited 20 times in total.
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Re: Undersealing A MK4

Postby mgrays » Tue Oct 27, 2015 2:25 pm

I would have stripped off front/rear bumpers and lights too (maybe next year..) .. it worries folk as damage/expensive if you get it wrong but there is often parts here that corrode badly that are not seen. Get a couple of plastic timber horses to put the bumpers on as they are too easy to damage otherwise; attached is a paint job mid stream.. the fug is the paint not the camera!
bumpers on timber horses.jpg
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