I made my facia out of Masonite, so I used a circular saw with a 40 tooth finish blade to cut the smooth bottom edge. For the curved edge at the top I use a jig saw and a high TPI blade.
All good advise, I also used a sanding block to round the edges off just a little to help keep the paint from chipping off. As an aside Masonite will soak up a couple coats of paint before the color is opaque. Will you have any variations in the width of your facia? Or will they all be the same? Reason why is my local true-value will cut a 4x8 sheet as thin as you want for a small fee, if all yours will be the same width. Please post photos of your efforts, we like to see what people are doing, we all can learn from one another, no matter how long we have been in the hobby. Regards,
