GRP Repairs Write Up Page

Issues To Deal With

Stuff You'll Need

Cracking Up

Grinding Down

A Tissue

Getting Pasted


Fibreglass, or GRP (Glass Reinforced Polyester) was discovered in the late 1940’s.

It was quickly adopted during the 1950’s and 60’s for a wide range of applications where its corrosive-proof properties allied with its high strength and excellent appearance soon proved to be invaluable.Its first main application was for boat building, where it gained acceptance in the 50’s and is still widely used today.

The uses for GRP have since broadened to the extent that it has now become the standard material for the construction of small craft, water tanks, building cladding panels, roof lights and of motor cars.

TVR recognised the value of the light weight properties of GRP and coupled with its then state-of-the-art space frame chassis made a very sound basis for building cars that were quick off the mark and fun to drive.

Anyone who remembers cars produced in the 60’s, 70’s and 80’s using the standard mild steel construction would know that the vast majority had one prime and very serious issue; rust.

I particularly recall being invited to stand next to friend to watch his newly acquired Austin Allegro rusting before our eyes.You couldn’t actually see it (obviously) but you knew for an absolute certainty it was going on.

A combination of poor quality steel, appalling manufacturing quality, shoddy painting techniques and a complete disregard for issues such as dis-similar metal electrolysis made for cars with a life expectancy of around 8 years on average.

These days the situation is very different, cars have evolved, in an almost Darwinian, natural selection fashion to what we have today.

GRP is still used, though production techniques have changed significantly the basic material still remains the same and still has similar issues.

This little section of my TVR S3 rebuild site deals with repairing the issues I had with my own car.

It does not serve as a guide and does not seek to come across as expert advice; far from it.

Before today (2nd September 2013) I had not attempted repair 1 and was still seeking the advice of a lot of literature (most notably; "How to restore fibre glass bodywork" by Miles Wilkins )

Issues to deal with.

There were a lot of cracks on the car, I wouldn’t say covered as I’m sure I’d have noticed this during the purchase and would have led me to believe that the car had been mis-handled (shunted or hard driven on rough ground) but that wasn’t the case.In the early 80’s I’d had a Taimar for a couple of years and, whilst there were a few cracks and a little crazing generally it was in top order and I loved driving it. (Much more interesting than the TR4a’s and 6’s that I’d had previously).

On getting the car home the chassis became the most important feature as the outriggers had almost completely turned to dust. This now being sorted attention turned to the body work and close inspection revealed around 15 areas of concern.All (the famous) ‘Gel cracks’ bar one small section near the offside front bumper where a previous driver had hit something a little more solid.

Stuff you'll need.

The kit below should be enough to correct many more gel cracks than you would expect to see on a single car.

That being said, my plan is to do all the necessary preparation work beforehand so that I don't have to mix chemicals over and over again, there's always waste so if you do it too often you may run out.

The kit includes both mat and surface tissue.

Mat is the rough stuff that I'm planning to use on the bulkhead repair by the passenger’s feet caused by the body lift.

As you pick the body off the chassis, once it's around 3 inches away either move the tub back or roll the chassis forward so that the bulkhead floor clears the chassis tubing around the transmission bell-housing.The resin gets mixed with the catalyst (between 1% and 2% catalyst to resin) using the syringe and the mixing buckets.

The acetone is for cleaning the brushes afterwards. Be sure to use the mixing buckets as the resin / catalyst reaction is exothermic so it heats up. Glass jars are a non-starter.



This kit, or similar will help with final profiling of the filled area...



… but bear in mind that each tool has a Velcro base except the black rubber block so you’ll need the equivalent paper….


Rubbing down will require various grits of wet and dry paper; available from many sources. I used 120, 320, 600
and 1000 grit

Try and get hold of a few cheap paint brushes, these are useful for spreading resin and for easing the tissue up to the edges of the repair patch.

Use the acetone for brush cleaning sparingly as it may have to go along way. Don’t use thinners as there’s a possibility of a reaction with the resin. Also, bear in mind that you shouldn’t use thinners for cleaning down any exposed i.e. unpainted areas of the body shell. Thinners will attack the resin and weaken the structure.

Cracking Up.

Gel cracks are usually associated with areas of stress on the car. Whilst my S3 had none around the doors and door handles (a common place in other cars, most notably the early Lotus Elan ) I did find significant crazing around the boot lid scuttle (near the solenoid mount) and on the bonnet.

The bonnet was quite interesting as the two ‘star’ cracks were directly over the two battery terminal tops.

Home - Up


This one is over a battery terminal. It looks like a scratch but it’s not.


This one is in that depression between the bonnet bulge and the wheel arch and very difficult to get to with and angle grinder.

Patience is your best ally here.


Grinding Down.

On reading the book it was suggested that I use a 5” angle grinder with pad and 80 grade disk to remove the paint, undercoat, primer and gel coat.

Knowing the extreme cutting power of the edge of one of these disks the initial approach was always going to be very careful. So with a touch like a mid-wife I eased the disc onto the paintwork, this proved to be the right method.

Dust is sent flying and before you know it your down to the beige lacquer with the GRP mat underneath.

Be careful here, because even though the cracks may have disappeared they will still be there unless you rip off the gel coat.

I tried wetting the area occasionally to see if the cracks were still visible but what proved best was to gently grind an area of paint nearby so that it flicked dust onto the cracked area; this show up the crazing nicely until it had all been ground out.



Casting a (not very experienced) forensic eye over the crack above it looks to me as though this was caused by an impact from the paintwork side, i.e. a stone chip perhaps.

You may find that to the touch, the ground out area is somewhat ridged where the disc has gone deeper in places, try to flatten this off as best you can with the grinder then take an orbital sander (120 grit) to it, you should end up with a reasonably feathered back area surrounding the exposed mat and gel coat.


The shot above is the same area as the visible gel star in the picture before that. As you can see, no crack lines.

A point to note here is that some sources of information, YouTube etc. will have you ‘routing’ out the cracks with a Stanley knife, filling with P40 and sanding down.

This is NOT a long term fix and demonstrates that the guys in the videos (usually burly Americans with lots of attitude and bad manners) have very little knowledge about what is going on underneath.

The filler isn’t nearly as strong as the gel coat and as the body flexes over time, the differences in material will slowly shift the filler and draw out the crack again.

Whereas, removing the crack completely, applying resin and fibreglass ‘tissue’ over ALL the affected area will normalise the structure making it sound again. Indeed, the book suggest that the repaired area will be much more reluctant to craze under stress than before.

 A Tissue.

Once the cracks have been ground away you can start thinking about the reapir itself. On closer inspection of the ground out area I felt that the depth profile, instead of being ...



was more ...



So, to prepare the tissue first I started by cutting out a shape that was around 10mm unside the feathered back area.

Scissors are ok for this but the fibres are really thin and very, very irritating on the skin.

So use the gloves supplied with the kit or better still find a box of neoprene gloves (the ones I have are in blue).

Also wear a long sleeve shirt with a button collar, goggles and most importantly a face mask.  


Initially I did 4 areas on the bonnet, 3 sections on the bonnet top and a section in the 'mouth'.

I also decided to try and even out the ground out section by using three pieces of concentric tissue, gradually getting smaller ...


This totalled an area of aroun 20cm2 and when you've done all the tissue cutting its time to prepare the resin.

Before you start:-

Have the tissues ready

Have the mixing tub, brush, resin, catalyst and acetone cleaner ready and to hand


  1. Put on your gloves and mask

  2. Put a little acetone into a small shallow tub

  3. The kit came with a syringe so I drew 5 full syringes (50ml) and dumped them into the plastic container

  4. Rinse the syringe through in the acetone

  5. Add 1ml (that's all - 2% of the used resin) of the catalyst to the resin in the plastic container

  6. Mix thoroughly with the brush (The mixture may change colour a little)

  7. Now working quickly but carefully, brush on a thin film of resin on the exposed mat surface of the area under repair

  8. Place the largest piece of tissue cut for this area onto the resin and the dab the brush onto the surface easing the tissue up to the edge of the primer

  9. Do the same all over the patch until the whole area is covered - DO NOT brush side to side, this will make the tissue slide about


Do the next three areas and then the first again with the next piece of tissue.

The first one should be dryish to the touch by now.

Go round each until all three areas have three patches applied.

Keep checking the state of the resin.

Once the phase change from a liquid to a solid has started the rate of change accelerates very quickly.

As soon as the resin becomes anything like a jelly throw it out and wash everything in acetone.

If you've not finished, mix up another batch.

The next 4 pictures show the positioning and sticking down of the first and second layers.


Marking Out

First Coat of Resin and Tissue

Laying on the Second Tissue

Final Edging


Getting Pasted

Once the tissue is dried nicley, which depends on how much catalyst you used, it's time to apply the filler. I used David's P38 or Isopon as it's sometimes called.

You may also see David's P40 for sale; this isn't for filling but more used for joining new sections together like a sort of GRP weld. P40 is a resin paste mixed with crushed glass fibre whereas P38 is a resin paste mixed with a chalk addative. both need a catalyst to harden tem off.

On the P38 tin it suggest a proportion of a golf ball sized lump of paste with a pea sized lump of hardener.

Once the quantities are doled out mix carefully, without being over vigorous.

It's important not to get any air trapped in the mix so use a folding motion like making a merangue (if you've made one...)

Spread carefull working from the centre out and cover the feathered edges of the repair again, trying to keep the air from underneath the surface.

Finally the paste should be just slightly proud of the surrounding body work to allow for sanding.

If you leave it too proud with alot of peaks and troughs then it's the devil to sand back,

Once spread out it should look a little like this.

Try to avoid air pockets as shown in the diagram below.

Chances are the bottom of the air pocket will be below te required hieght of the repair and if you keep to the original profile (as of course you should) then the blemish will show.



Once sanded it should look something like this...


The middle patch looks ok, but once a hand was carefully run over the top it was obvious that the centre didn't have enough filler in it so more will have to be applied.