You state ‘best off letting the professionals do it’. Maybe, but not necessarily! I’ve seen pics of garages spraying underneath and they look a right mess. You only have to read the thread entitled ‘Undersealing at MX5 City in 2020’ to appreciate what can (and did) happen. As I mentioned earlier in the thread I have 80-odd percent prepared the wheel arches (and all their components) so easy enough to rust treat and apply underseal in those areas - which I will complete myself. If I can’t gain enough height with car ramps, then I’ll get the car on the hoist/ramp at my local garage to treat the underside.
One thing to remember is you can do the job as slowly as you like. Forward rear arch area on an nc would be first along with inside the sills. The rest can be done in stages as weather or time allows. If you have a couple of aerosols of bilt-hamber ub and some s-50 for the cavities. The stuff stores well in the can and the nozzles and injection lances can be blown through with a quick shot of brake/parts cleaner ready for next time. I usually check and re-treat some areas when doing other jobs like brake pads, oil change etc.
2 Likes
Chris. What would that type of work cost, and would a normal garage do it or is it a specialist job? I’d guess the younger the car the better really?
Love this post 
my mk3 has been garaged since Nov so no salt. However changing out suspension so going to remove any rust and underseal myself. Some good comments and suggestions on products.
I am getting a new ND in March. I want to get it undersealed as I plan on keeping it, but even if I don’t, I want it to be ok for future users.
I am therefore looking at getting it undersealed. It is coming with factory lowered suspension and I will not be able to easily get underneath it.
I live in the NW and as this will be my only car, I have not found anyone in the local area where I could get them to do the job reasonably quickly.
Therefore can I do it myself and what advice is there for a systematic way without me having to take a brand new car apart.
I know there will be different answers, but is there either somewhere near Cheshire that could do it (properly) or is there a “minimum” that I can do that would help…
thanks
I’ve had estimates ranging from £350 to around £500!! Definitely doing it myself…in manageable phases as it’s time consuming.
Hi S,
I used this Mk4 Underseal How To - Mazda-Menders as a guide and found it invaluable.
Basic principal of doing it yourself is ‘if you see a hole get some waxoil in there’.
If you are willing to invest in tools get a small compressor and you will have loads of fun. I know I did and still do.
2 Likes
Thanks…
that is a very involved process to do properly and unless you have a lift, is impossible (or very difficult).
I can see why it is expensive to get done properly. would rather it was done looking at the pictures as it looks very exposed.
Very disappointed that the cars are allowed to leave the factory for markets that use salt on the roads, without adequate protection.
1 Like
I’m the opposite. I have worked on many motorcycles, cleaning stuff up, painting, restoration etc.
That kind of stuff is very time consuming and extremely messy. If you want it done right then you really need the car on a lift and to spend many days cleaning up underneath before applying the underseal. (Depending on the current condition)
A professional will know what underseal is best. Many use a rubberised underseal which is very strong vs debris hitting underneath the car, they also know which underseals not to use because some do hold moisture and create pockets where rust will then not be visible at first and when it is visible, it will be rotting.
I’m not saying the guy at home can’t do a good job, it’s not rocket science after all, it’s just there is a right way of doing it. If you have to space to do it then go for it but I will be paying a professional body shop to do it.
They quoted me £410 all in with cavity wax, that’s all the arch liners removed, wheels taken off, undertray removed etc
2 Likes
I disagree with you, given the number of internet tutorials and videos about the subject I would argue that anyone who is reasonably competent could do a far better job themselves. Rubberised and bitumen based compounds are not required as most if not all of them hide the problem. A set of axle stands and a decent jack will raise the car to the height required. It does depend on the amount of remediation work necessary but again there are good products out there for diy use.
1 Like
I did mine. All covers and wheels off. Shocks coils and arbs off. On axle stands. But it is messy. Very messy. First cleaning, very dirty, got absolutely soaked. Removing rust is hard work and the use of rust converter can be nasty. Choose your treatment, I used a waxoyl based underseal, but it goes everywhere and then a protector like waxoyl is horrid.
Without a lift you are working on your back and often your face is very close to the job and you don’t have full arm extension available.
But when it’s done you know exactly what you’ve done. Also gives chance to clean the covers and other protective pieces and really get to know your car.
Not a job for the faint hearted and, please wear goggles.
Having said all this, I would still do my own again…
1 Like
On this topic, what’s the safest way to raise the car to be able to get underneath it to undertake such tasks as undersealing?
4 axles stands and a trolley jack?
An expensive platform lift?
1 Like
Four axle stands minimum. Some would still not go under even. Leave some weight on the jack maybe, but will restrict access.
2 Likes
Thanks.
Would axles stands be high enough to gain enough access underneath?
Good point about “4 axle stands minimum”. What number is sensible? 6 or 8?
It worked for me and I was ok with four good stands. I had sufficient clearance. I also used ramps when I could, obviously when wheels were back on.
1 Like
How many ramps, 2 or 4? 
Two, front or rear, then opposite on stands
1 Like
I wouldn’t attempt a DIY job without access to some kind of lift. One of the MK4’s referenced in the Mazda Menders article above is my first one (we did the same on the current one) and to effectively get all the covers off and into all the spaces and voids you’d want to then axle stands don’t really cut it.
3 Likes
My comments relate to Mk3
2 Likes
I used the hammerite waxoyl spray with the car on stands but done this when I was doing my rear arches and sills, so had the rear bumber off which gave good access to a lot of hidden areas. Done a top up when I had access to a ramp but was still very good even 3 years later. To be fair though I do avoid driving in the rain!!
1 Like