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[LLBenJ]

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[Desert Sasqwatch]

Dremel tool with the sanding cylinder would be my go to tool - and a dust mask.

 

[Desert Sasqwatch]

A diamond wheel works well for fiberglass. Harbor Freight has these for the Dremel tool for pretty cheap.

 

[Andy]

high speed cutting wheel tape the edges to try and prevent splintering. If it does splinter you can glue it back down with super glue or epoxy.

 

[GoblinGuyZ]

A bigger wheel and a slower rpm are your friend, a small fast wheel will be harder to get a nice even smooth curve as you try to follow your marked line. And x2 on the dust mask, fiberglass shavings are no joke

 

[Robinjo]

I just trimmed mine with a Harbour Freight grinder and a cutting wheel. I used a 3 or 4 inch cutting wheel and just controlled the speed to be not too fast. It cut it very quickly. I do agree with GoblinGuyZ, wear eye protection and a mask. It makes a lot of dust.