I'm about to paint/re-caulk the walls above a fiberglass shower. There's a ledge that I'd just as soon eliminate by putting in something to slope and prevent water from puddling in the corner (shown in photo). What can I use to fill the void?
Short of recasting the tub I can only think of one solution: Water always flows downhill, so if you can get some shims under that end of the tub to elevate it enough that water flows away from the wall you're set. Otherwise invest in a squeegee and wipe the ledge diligently after using the shower.
You can buy some polyfill which is basically a two part epoxy used to replace coatings on surrounds and tubs. It is pretty thick stuff so you would be able to add it to this until you sloped away. However I am not sure how much you can put on at once (I have never tried beyond normal tub repair) and I am not sure how thick it can be in total.
As already stated, this is a problem with the mould and you cannot really correct it. The only thing that i would try, is to chip away the existing silicon and cement around the corners of the ledge and then try to get something which you could use to pry the ledge upwards ever so slightly.
What I would use is some expanding foam, you will be surprised the force that it can apply (provided you can restrict the flow back out the sides of the ledge)
The other thing to try, is to build up those indented corners slightly so that there is a smooth transition to the wall, that way, at least the runoff water will push existing water off the ledge and will not pool in those sections.