After finding advice online I decided to risk painting my tatty, second hand, poly cotton tent. I used fabric paints (Dylon make them and there are other brands) after I painted on the design it was left to dry, and then I ironed each painted section of the tent on the hottest setting the tent would stand to fix the dye. Once dried and ironed I sprayed over each painted area with Fabsil waterproofer (you can see what looks like a wet patch around the painted areas, it dries clear and invisible) and now all I need to do is test the tent in the rain!
Wish us luck! The tent will make it's debut at Camp Bestival this year.