I've a small patching job (a hole about 60mm) on a fibro sheet.
I'd thought to glue a patch on the back of the hole and fill it from the front. What glue will stick to fibro for this purpose?
Or you could probably do it with builders bog and also fill the majority of the hole at the same time ... then just top coat. Just put a little screw or nail in the middle of the patch to hold it while the glue dries (if it's too heavy to stay there itself).
patching fibro 
