How can I waterproof my asbestos Nissen Hut?

Our asbestos Nissen hut is permanaently damp. I'm going to address the rising damp problem by laying a DPM and a couple of inches of screed to at least dry, level and flatten the floor.

There are some obvious holes and gaps between panels, which I will be fixing, but is there any way to waterproof the outside of the asbestos so that it will dry out to the extent that I can at least paint it (on the inside!) without the paint flaking off as appears to have happened to previous attempts?

They are many bitumen type products to paint over the sheet and the only problem is they are mostly in black colour.

Try asking on the Decorating / Painting / Wall Tiling, they may have a better idea.