De BISAWiki
IDooky</a>. I wipe out the excess mud, just as I would with paper tape.
When this first layer of tape is challenging, then I lay down a line of paper tape proper across the fiberglass and centered on the underlying crack. Now I have double insurance: the glass for lateral strength and the paper to maintain it tight to the surface. This usually requires care of such cracks, or any cracks you are particularly concerned about. (Never forget two or more topping coats.)
I do the exact same operation or a variant of it on bad plaster cracks.
A single final point. I've emphasized toughness of the compound you use. For that cause, I like "hot mud", a setting sort joint compound far more like plaster for strength. If you are unsure of yourself and your speed, it would be very good to use hot mud with a longer setting time, like an hour or 90 minutes. Mix tiny batches and do not forget to wash your tools and pans nicely between batches.
