Revisions log for Answer #1111

 I'm afraid that there is no way to stop the process of burning out ondulin. Ondulin is pressed waste paper plus fiberglass, which after pressing is dyed and then impregnated with purified bitumen. As manufacturers assure us, dyes are organic, ie by their nature of light-resistant, but think about it, where is it so much organic to pick up, to hundreds of tons of manufactured products to paint? Most likely the first batch really were with organic dyes, and then the usual principle of the market: "do not cheat – do not sell. Unfortunately, the process of decay of pigments from the action of ultraviolet rays can neither be prevented nor restored. Theoretically, there is an acrylic varnish with UV filters, but it is not designed for ondulin. Try some small area of the roof they cover, if ondulin "befriend" with varnish and stop burning out then you can and then paint the whole roof. But only I've seen on sale this varnish toned brown.