Yes. Cracked gel coat, star cracks and even a split or a hole in the fibreglass itself can be repaired and painted. The order matters: the damage is ground back until no crack is left, the fibreglass is built back up where it needs it, and only then is it primed with a primer made to go on fibreglass and painted.
Paint over a crack that's still there, or over a panel that still flexes, and the crack shows through the new paint in time.
Where does fibreglass turn up on cars and campers?
On kit cars, on classics built with fibreglass bodies, on replacement bonnets, wings and spoilers, and on camper roofs, where plenty of high tops and pop-tops are made of it. Fibreglass is glass fibre matting set in resin, and the trade often calls it GRP, short for glass reinforced plastic.
If you're not sure what a panel is made of, a magnet won't stick to fibreglass, though it won't stick to aluminium either. The back of a fibreglass panel usually shows the rough texture of the glass matting.
What is gel coat?
The hard, smooth outer layer. Scott Bader, a UK resin maker, describes gel coat in its guide to common gelcoat faults as the outermost resin layer applied to many composite parts, giving both a smooth, glossy surface and protection. The laminate, the layers of glass matting and resin, sits behind it and gives the panel its strength.
What causes star cracks and crazing?
Star cracks are fine cracks running out from one point, like a star. Practical Boat Owner's note on star crazing says they usually radiate from a point where an excessive load has deformed the surface. On a car or camper that can be a knock, a stone or a weight leant on the panel. Crazing is a web of fine lines over a wider area.
Scott Bader adds that cracks and crazing can come from gel coat laid on too thick, rapid temperature changes or uneven curing, and that over time they can reduce both the strength of the part and its looks.
Can star cracks just be filled and painted over?
No. Scott Bader's gelcoat repair guide starts by asking whether the gel coat is cracked, or the laminate below it too. Where the damage carries on into the laminate, it says to keep going until there are no signs of cracking in the laminate.
So each crack is ground or sanded out to sound material before anything goes back on.
How is damaged fibreglass rebuilt?
With new glass matting and resin. Where a panel is split, holed or cracked right through, the broken fibreglass is ground back to sound laminate and new layers are laid in to build it back to strength, from behind wherever the back of the panel can be reached. Then it's filled and blocked flat like any other panel, with a long sanding block over a guide coat, a light dusting that stays in the low spots. See bodywork for how a panel is made flat before paint.
Why do some cracks keep coming back?
Because the panel flexes there. Practical Boat Owner says that if star crazing comes from normal loads, the structure is too flexible and is best backed with extra stiffening. A crack that comes back in the same place after a repair is the sign. The cure is stiffening the panel from behind, not more filler on the front.
Does fibreglass need its own primer?
It needs a primer made to go on it. Paint makers say which surfaces each primer is for: Standox lists GRP alongside cured paint, polyester body fillers and prepared metals for its High Build Primer Filler. Before the primer goes on, the fibreglass is keyed, sanded with a fine scratch for the primer to grip.
Will a fibreglass panel match the rest of the car?
It should. Once it's primed, the colour and lacquer go on the same as on steel, mixed to the car's paint code and blended into the panels beside it so the join doesn't show. On a camper or a car that needs paint all over, see resprays.
What moves the cost?
How far the damage goes, whether the fibreglass itself needs rebuilding or stiffening, whether the back of the panel can be reached, and how many panels need paint. A few star cracks in one panel are a smaller job than a split roof. We give a rough price from photos, confirm it once the vehicle is here and the cracks are ground out, and show you anything we find. See prices for how we quote.
Cracked fibreglass? Send us photos of the damage close up and from a few steps back from the quote form.
