Why quality of product?
All you need to do is provide some token means of filtration within the solar melter (say - paper tissue, or open-weave fabric) to create an efficient single-pass operation. Some people even manage to achieve a fine product without such filtration.
The crud (slum-gum) always stays behind - so that when the melter has cooled, this can then be dumped. The quality of wax recovered from a solar melter can be close to 100% pure, and thus can be used 'as is' or sold without any further processing.
In contrast - with boiling, you need to separate the slum-gum by hand during the process. But some of this 'crud' is always retained within the wax, along with propolis and light particle dirt(*) - so purification of the wax often then becomes a two or even three-stage process.
(*) The FatBeeMan has posted some YouTube videos related to his method of recovering wax - I'm pretty sure his process is 3 or even 4 stage.
LJ