Best Filler to Use?

[MrBounce]

I am about to start thinking about sorting out the cracks on the front of my project. Now once they have been scraped/ground out they will need filling in. Not being 100% sure of fibreglass, before I get started I just want to make sure I am about to do the right thing

Depending on a) how deep the groove is and b) who you listen to, I can either fill with resin and then use filler, or just use filler.

Which is the best way? And what is the best filler to use?

[ACourtney]

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First you need to grind the cracks right out. Sometimes the cracks will just be in the gel, but often they will be in the laminate as well.
You can see where the laminate is damaged as it will have turned white and all the damaged area needs to be cut back. If you have long cracks, then sometimes you should repair the laminate in short sections, so as not to weaken the panel.

The laminate needs to be repaired with glass and resin. Very small cracks can be repaired with bridging filler such as Isopon P40 (glass strands in polyester resin), but larger areas of damage, with multiple cracks, will need to be re-laminated with CSM and resin. Sometimes the repairs need to be laminated in from both sides.

If you intend to paint the repaired area then the choice of filler to go over the repair is not too critical and it won't matter if some of your laminate shows through. In fact I tend to maximise the laminate by using a layer of glass tissue to overlap the feathered back gelcoat. Then I just use a skim of filler over the top. My preference has always been for metal based fillers, such as Chemical Metal or Metalik, as these don't take in moisture and expand. However, normal filler, i.e a talc loaded filler like P38 or Easysand, should be okay if you intend to use an impervious undercoat such as an epoxy.

It is possible to repair the gelcoat by applying new gel into the groove, but very difficult to get a good match. I would only try this on a relatively new panel and if I had some of the original gelcoat mix.

Looking at the photo's of your car on your rebuild thread, I can see that you are going to be busy. My son James has started on rebuilding a Mk1, which has a similar array of cracks on the front as well as the standard Mk1 bonnet stars, so we will be doing the same jobs sometime ourselves.