fiberglassing flat areas?

I would fleece it, otherwise there will be nothing to absorb the resin and you'll be left filling and sanding for hours to try to get the inperfections out (personal experiance).  The staples should be lower then the top of the fleece so you shouldn't have to deal with them once you glass the fleece.Kevin Pierson

It's usually much easier to use a piece of wood for the flat areas instead of fiberglass mat. If nothing else, then just for the cost alone. For instance say you're trying to make a 24" x 36" flat panel: you could use either an MDF (or plywood/birch/oak/etc) board which is very easy to cut to shape, or you could lay out the fiberglass mat & take forever laying layer upon layer to get the required thickness...the wood is a much cheaper & faster solution with the same result. I'm not a fan of glassing all sections just for the sake of using fiberglass, but I've known plenty who prefer to make an entire structure from fiberglass.     Flat areas flex more than curves, even if it's only an amprack you want more than a layer or 2 of mat so the panel doesn't break in half. If you're set on glassing the flat area, then there is an advantage to KPierson's suggestion - the fleece provides a surface to build up layers of fiberglass mat. Another possibility is to not use fleece, but use very large sections of woven mat instead of chop mat. I'm partial to chop mat when working with awkward shapes & areas, but woven mat is excellent for flat areas & it is very easy to layer thick.     As for your corners, I'd need some more info on what exactly you mean. Normally when you glass a project you either work in a negative space (a wheel well or other caved-in shape) which already has the rounded corners, or you build off of a frame. If you're glassing on a wood frame then you can rabbit the edges with a router so the edge you staple your fleece to sits a slight bit lower than the regular plane...then you can just use a roundover bit on the actual edge.

you can just use bodyfiller to cover flat areas of mdf.  i you use fleece and resin there is a good change it will shrink and deform the flat areas.