fiberglass2

PART TWO - Causes and Prevention of Damage

Every GRP car on the road today has a gel crack on it or at least a blemish in the GRP paintwork. Even the best prepared car at a concours event, or a brand new car even, will have a mark somewhere if you look close enough! Gel cracks are really GRPs answer to metal rust and, like rust, most of them should never appear. So apart from the obvious accident damage (about which more later) how do gel cracks appear, and why, and what can be done to prevent them happening in the first place?

Starting at the beginning, causes are from the mould itself. If the mould has been constructed from GRP (be it epoxy or polyester resin) and has an in-built crack in it or any other blemishes, then the pattern of the crack will be transferred to the moulding and will show as 'crack-indentation'. this is fairly common on, dare I say it, low budget kit cars and cheap replacement wings, bonnets etc for Morris Minors and the like. At least with the replacement parts the offending mark can be flatted off with wet and dry paper and then the whole panel primed and painted but, with self-coloured panels (where the colour is in the gel coat layer) then the blemish is there for all to see and if it is on, say, a flat bonnet area it will stick out like a sore thumb, much to the detriment of your pristine car. the only recourse is to paint out the fault and get the manufacturers to rectify the mould or make another one. (The classic example, as retorted to me when I admonished one poor kit car owner about the shocking state of a door panel was 'Oh, they're all like that"!)

From the mould itself we move on to the next stage where the moulding is released from the mould. If for any reason the moulding has not fully cured (i.e. it is still green) or has stuck fast because the release agent failed to release and too much force is applied, stress lines and cracks will appear and will show up as white lines. Then, if left unchecked, they immediately show through the paint that is applied subsequently. The remedy here is to make sure all the release agent is evenly applied - especially into any awkward areas - and that the moulding has fully cured before attempting to release it. It may also be that the mould design itself has been constructed in the wrong way to allow an easy release from the moulding - i.e. too sharp a radius on the corners, overhanging angles, double curvatures, etc. If necessary a redesign must be done to prevent inherent cracks in the moulding.

With a crack free moulding released, in built stresses may eventually appear as a crack, especially on hard butt edges where not enough thickness has been applied. Unless bonded to GRP correctly different materials, such as plywood or steel bulkheads, can move, causing stresses on the GRP. A thick laminate joining a thin flexible panel or a door panel beefed up around a hinge point will cause a gel crack to appear in time where the thin panel flexes and the stiffer hinge panel doesn't. the remedy on any panel is to grade the weight of mat accordingly, rising evenly to a maximum around the stress point. Sadly too many new GRP car manufacturers don't even bother and the factory laminators just slop it on any old way. If the cars were built correctly then they would be virtually bomb-proof and, having repaired just about every type of GRP car, I feel that so much could be done to prevent totally unnecessary damage,

Moving to the vehicle itself, what can be done to prevent cracks? The main problems are due to the high stress points i.e. door handles, hinges etc. and panel flexing due to a stiff bulkhead flexing the panel around it. This also applies to glass windscreens, be it front or rear, where the glass is heavy and does not give - but the GRP panel will. All fittings on any panel must have the load spread over as large an area as possible, especially around the door and boot handles. If necessary add one or two extra laminates behind the handle and grade them over an area, then use large 'penny' washers when bolting up. With all fittings DO NOT bolt up like a gorilla. Use common sense otherwise that sickening cracking sound will be heard and, yes, there is a crack on your brand new paint. Make sure with any panel that it fits correctly i.e. all doors, bonnets and boot lids fit their shut lines, since tugging away on the handle trying to open the door will only succeed in cracking the panel. A correctly fitting panel will require no force to shut or open it but force, wherever it is applied, will lead to cracks. NEVER slam doors and lids because cracks will appear over time.

Heavily stressed area

With bulkheads and glass windscreens, build up the area around the aperture to prevent flexing if required, but grade it away to the original thickness otherwise it will crack somewhere else.

Fitting trim and badges on a newly painted body is another cause of cracks and heartache. Drill the rivet or screw hole OVERSIZE i.e. if using 1/8 rivets use a 1/4 in drill; DO NOT force in a rivet or screw because as soon as you tighten it up - bang - a star crack will appear through the paint around the fixing. If possible always use a washer behind the rivet or screw to spread the load. With badges use double sided adhesive tape if possible to save using screws or rivets. Remember any mark will remain for life and may spread. The only way to get rid of it is to do a repair and repaint. Fitting the 'pretty' bits causes more damage to new GRP bodies than any other operation.

Other areas to watch are the fitting on non standard items like sun roofs, fancy air cleaners etc. and not realising that they are chaffing on the adjacent GRP panels, thus causing damage. With any item do not force a panel over it, always check first that it does no foul before shutting the panel. with any exhaust system, wiring and water pipes make sure there is a large enough hole so they don't touch and cause cracks. With any wiring always use a rubber grommet to prevent the raw edge cutting into the wires. Make sure under panels, such as wheelarches, are well protected by an anti stone chip paint (3M supply a good one) or a good FLEXIBLE underbody seal so that stones flung up by the wheel do not star crack the GRP from underneath (Lotus Elans and Europas suffer from this).

Most, if not all, of the foregoing can usually be avoided with simple precautions and attention to detail. The next cause cannot! It is that of accident damage. Proper GRP cars (such as Reliant, Lotus, TVR and Marcos) are infinitely better in an accident than metal cars because GRP will absorb the impact and shatter, NOT deform, thus minimising panel damage. Metal will deform and go on deforming i.e. a front panel through the wing, A post, roof, door etc and be a total write off.

Furthermore, pulling out metal bodywork, aligning on jigs and welding on new panels actually puts more stress into the bodyshell, whereas bonding on a new GRP panel should cause NO stress at all. After a major accident the extent of the gel crazing may not be evident for some days or even weeks later, when it shows. Driving a car after a minor accident will cause cracking to spread, making life difficult for any accident estimator to assess the TRUE damage. I always look for the slight deflections of light over the paintwork to tell me where the cracking stops, even though nobody else can even see the cracks beneath the paint. It goes without saying that all accident damage/crazing must be removed completely to prevent any further stress/gel cracks occurring again on the 'repaired' area. All too often new sections are bonded in and 6 inches behind the joint all that is under the new paint are the remains of gel cracking from the accident. If new sections are bonded in it is essential that the thickness of the new laminate matches the old and the join itself must be graded over a wide area.

Other causes of GRP damage, apart from fire, are confined to nature. GRP will delaminate if left out for years with no protection; water will enter the mat strands which act like capillaries, and, if there is a severe frost, the layers can physically separate. Once water is in GRP then it must be dried out in an oven for about a week at 85/100 deg F before any attempt at repairs or painting can take place otherwise water will just break through, giving unsightly bubbles and blisters on the paint surface.

Severe heat will distort the panel or bodyshell. Above 140 deg F the resin MAY start to flow and, once set again, the deformed panel will have to be replaced. This is why one NEVER low bakes GRP bodies when painting (however many people still try it with disastrous results).

Chemical damage is confined to paint strippers (methylene chloride) , hydrofluoric acid (glass etching) and certain fuels over a period will leech out the resin (after all resin is derived from fuel).

Paint strippers will eat into the gel coat and resin and, if left, will make a hole. The only action is to cut out and repair. Hydroflouric acid not only will eat into the GRP but will eat into you as well! HF is NOT available to the public due to its lethal qualities. Brake fluids and anti-freezes do not attack GRP. Fire is the most feared and any fire damage on GRP MUST be cut away since only the glass mat is left. The mat has no strength at all after the resin has burnt away. Never bond new panels to fire damaged old ones because, at best, you will make a very weak bond.

GRP is pretty indestructible and really will last a lifetime if looked after correctly.