Beautiful work Matti, 2 weeks ago i cavity waxed all hollow sections of my shell, i had to get creative to find ways inside and drill a few holes, as you say its a messy job, its still dripping out, good job i covered the floor with cardboard, that stuff gets everywhere but it has to be done.
I followed the link you posted, its the 4mm bitumen i need right ? how many of the sheets should i order to do a correct job like you did ?