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Fibreglass bonnet repair


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My bonnet from eBay had lots of little "spider" cracks in the gelcoat which I suppose were stress cracks. I hit them all with my grinder and went back to fibreglass in all these areas, thankful to see no cracks in the glass itself (gelcoat must have gotten brittle with age).

Anyway, I tried laying fibreglass sheet onto the areas i'd taken back to glass, however once dry it was near impossible for me to hand sand ripping up even 40 grit sandpaper instantly. This has led me to wonder what's the best way to repair these "crevasses" in my bonnet? I was thinking just applying and layering bondo until I got the desired height then sanding flat? I do have some fibreglass filler if this would be any better? I did try filling them up with just resin in some areas and this seems to have taken most of the depth away, so shall I do this on the rest?

Any helps appreciated,


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Star cracks are the result of 'dings' that would have left a small dent ina steel panel, but as GRP is inherently flexible, the glass rebounds, but the gelcoat, with no fibres in it, does not and cracks.   The fibres in the deeper layers break and de-laminate, leaving a weak area.    Just repairing the surface is not going to solve it, as the weakened panel will flex and crack the repair.

Instead, mark the area inside, by drilling right through at the extremities of each of the star's arms.  This will also inhibit extension of the cracks.  Then on the reverse side, grind off the dirty surface widely around these marks and reinforce with several layers of new mat and resin.  Then grind out the cracks, no need to do the whole surface, and fill with body filler, before sanding back to fair surface.

Now, the panel is stabilised and its strength restorred, so that the new filler will stay there and the star wil not reappear.  IT can be a laborious and time consuming job doing this to an old an damaged panel!

Good luck!

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