Mazda Rotary Club Wiki

for all rotary enthusiasts

User Tools

Site Tools


RX-8 Technical Articles








Lubricants & Fluids





Bodywork & Styling


Fuel System

Competition Upgrades



Well somone hit my car the other night and left some nice work for me to do. Here is the damage I was greeted by in the morning:

So after having to repair my bumper less than 6 months ago i find i am having to do it again… this time with the knowledge that i know what i am doing.

Well till it comes to painting it that it.

So this is more of a rough guide than a full on step by step DIY… it should give you the right idea though.

Remember when working with Fiberglass, cover up, wear a mask and goggles and some latex gloves are a good idea too. spceially when it comes to mixing up the resin.

I removed the bumper to be greated by some nice damage to the rear though the back does not look as bad as the front:

There seems to be another small crack towards the centre however this is not visable from the front so i plan to just clean the back up and lay fomr fiberglass in there, to strengthen it up.

I then stripped the bumper of all its grills and brackets in the area i am going to repair.

The next thing your going to need to do it drill some holes at the end of all the cracks to stop them going any further, this is where is starts to look worst before it looks better.

Once thats done you are going to need to start cutting out all the broken and damaged fibers. I prefer to use a small dremel grinding wheel for this part and start by “dishing” the front out along where all the cracks are. It does not show it in the picture but look close enough and you can trace the lines where the cracks are going.

After this you are going to need to get heavy with it and cut some gaping holes in the material to get fid of any left over material thats broken. As you can see i have started “dishing” out the the inside of the mouth area too in this picture, this will eventually be done on all of the cuts and holes.

This picture shows where I have started to clean up the back of the other bit of the damage. What you may sometimes find is that the fibre will delaminate from itself and while you think you have cut it all out there is areas where the fibers have split in parallel with the surface of the bumper. These really do need to be cut out dont be shy with that dremel.

Another Image of the front being cleaned up.

The Next strp is to start prepping it for fiberglassing and resin… so stop all the resin dripping out the gaps when i apply it I have covered all the holes with “Velocity Red” Gaffa tape. This also helps keep everything in line and you will see in some later pictures that I use some metal bar with some carpet tape to line everything back up again.

Some scrap ally clamped along the back edge of the mouth helps keep everything in line. I have alsy masked off most of the bumper with take and plastic to prevent unwanted resin dripping on parts that wont need painting again.

This image shows where I have keyed the surface up with an angle grinder, this gets rid of loose dirt and fibers it also helps remove the paint from the inside arounf the area we are going to be repairing. Once I have cleaned the surface I also added some shallow scoring in a criss cross manner to help the new resin and glass to adhere better.

I had to skip the various stages of glassing it out due to having fingers covered in rapidly hardening resin. Your going to want some throw away tin trays for this stage as your going to need to soak your matting in the resin before applying it to the damaged area. Cut your fiberglass matting to the sizes your going to need before you start mixing the resin and hardener. Mix your resin to the correct ratios on the packaging, I have used plastic padding here as its what i have used before.

Once your resin is mixed start by brushing on some resin with some of the resin with a sheap paint brush, your going to need to work fast so plan in advance where your going to place your matting.

You want to start with some large pieces over the damaged area, then work out from there overlapping then where you can to help build strength. once you have laid your wet matting on to the area your doing to need to add more resin to the back to stick it in plate.


This will just pull your fibers apart… add resin to the back with a stippeling action making sure you push the wet matting in to any corners and cracks.

Keep building up layers as long as your previous layers are still wet, make sure you do not lift any of the fibers up and breathe air pockets in your repair. if you do and it sets hard then you will need to sand these out and add more resin and matting once everything is hard (this takes more time).

All is left to do then is allow your repair to set.

Heat helps the mixture of resin and hardener set quicker so here I am using a halogen heater.

Once it's all set hard next I'll remove all the coverings from the front and start work on cleaning it up ready for bodyfiller and eventually painting.

After the Fiberglass had set hard i removed all of the masking and Gaffa tape from the front of the bumper, The heat has left some of the glue stuck to the paintwork and some of the tape has also been grabbed by the Resin as it set, this it no big problem as the stuff stuck to the resin will be removes in the next step.

You can see from the pictures that there are areas that the Matting has not gotten right down in to, These could leave air pockets when we apply the body filler if left as is. These will need to be ground out with the dremel and a grinding wheel, leaving a smooth surface to apply the filler in to. There are several areas like this on the repair.

The next picture shows what it looks like once we start cleaning up the area ready for filler. This will help smooth out any highs and sharp edges or “caves” between the damaged area and the new matting applied to the repair. You can also see from the pictures the cracking of the surface paint caused by the fool that couldn't drive.

These are most of the tools we are gonna need to finish this part of the job:

  • Some form of Body Filler (preferably easy to sand)
  • Hardener (usually comes with the filler but a good idea to get extra)
  • Random Orbital Sander
  • 80grit, 120grit and 240grit Sanding discs
  • Body Filler Applicators (these ones were silverline and made from springy steal so follow the contours better)
  • Something flat to mix your bodyfiller on (I use a bit of cut up posterboard)
  • Air Duster (attached to compressor)
  • Brake Cleaner (not pictures)

I started to Sand out the areas to be filled using the orbital sander and some 80grit discs, this cuts through the paint and srein quite easily, you need to smooth any really high ridges out that are not in line with your finished surface.

Just working it out a little more

Next you are going to want to clean the area up of all the dust and old glue this is where the Brake cleaner comes it.

Start by blowing as much of the dust ect away with the air duster attachment then give the area a good dose of brake cleaner, then sipe clean with a dlean Lint free cloth. The brake cleaner should help remove any glue thats stuck to the paint in places. Once its clean give it one more blast of brake cleaner then dry it all off with the compressed air from the air duster.


This is never a one step process annoyingly, start by mixing a small amount of the body filler putty with the correct ratio of hardner mis well as per the instructions. Now as with the resin earlier your going to need to work fast but have a plan of attack.

Start by applying a small amount to the deep areas and making sure its pushed well in. then you can add extra to smooth the surface, your probably going to get a fed low spots but this can be sorted with another application of filler later on.

Just keep working with whatever size applicator card suite the area. When finishing the surface work in slot smooth motions in one direction making sure your applicatior is wide enough to bridge the gaps your filling and can also follow the contours of the undamaged body.

When it some to corners this is where it takes come time, its generally a case of applying some to fill the area than danding it back once hard then applying more till you can build up enough filler to then shape your corner of curve with sand paper etc.

The above picture shows the first application of filler for the leading edge of the mouth I'll need to sand this back then build up more to achieve the desired shape.

While the second dose of body filler sets lets fill you in on what your missing.

Well now that the Filler has set, it dont take long, maybe 20 minutes. It's just a matter of hitting it with the orbital sander, I am still using the 80grit discs will it comes to finishing, at this stage its just about filling the dips in the repair then danding everything back flat and smooth again.

The lower section is a little more tricky as its a far more complex shape however it's much the same, it's just all about sanding away all the ridges and excess filler till its getting towards the shape you need. As you can see from the picture there were a few air pockets that got trapped in the first layer, however I am quite lucky as they were mostly trapped in excess filler that requires removing anyway.

And a little more work this time with some 320grit paper on a block of wood. just taking as much of the excess off as possible, and returning the body back in to its correct shape.

I had a few sharp ridges of filler in places these I had to gently sand doen with 320grit by hand (finger) no block, I also used the Orbital sander in plaved that needed feathering out to a smooth edge to meet with the original surface.

Clean off the excess dust from sanding ready for more filler.

Next up its time to mix up a little more filler, for the second skim coat, The idea now is to start filling in the gaps left by sanding the first coat and try to build up any edges so you can shape them later on.

With the plastic padding your mixing ration is roughly a gold ball size blob of putty to a pea size blob of hardener (the more hardener you add the quicker it will go off).

Again with the applicators start to fill the low spots, with this coat your going to try and build up a little extra thickness so hopefully you can sand it back to a nice smooth finish, don't worry if you need to add further body filler, just keep doing it till your happy with the finish.

When the filer starts to pull like in this picture ist starting to set, so you have not got to leave it to go off sand it back and add some more if needed.

So the second skim coats set and i have started sanding it down.

Not far off the right shape for the bottom edge nowm just a couple of small low spots to fill again.

Flatting the top part of the damage out with the orbital sander here… and now feeling with my finders for any small high or low spots that will need sanding smooth

Not far away now.

Now its time to clean everything up again, for a third and hopefully last coat of filler. there is a small low spot i missed with the second coat at the top point of the high damaged ares this is quite deep to needs filler for sure.

I am going to leave this to go off overnight… not realy because it needs to but because its late and i have been on this for the best poart of the day.

Well this morning I got to rubbing the last layer of body filler down and smoothing things out ready for *cough* Paint *cough* I have decided that with brands just round the corner than ill just wrap it in some Vinyl to protect it and possibly make it look a little more respectable. Once i am ready for paint ill remove the Vinyl and key everything up ready for painting.

anyway here are a couple of pictures of the final job.

technical/rx-8/bumperrepair.txt · Last modified: 2014/05/11 17:48 by crissy