I reside in a 15 year old flat roof covering 1400 sq. ft. of my house. The roof covering is a black rubber membrane. It has never been kept up with so there is cracking and some leaking. I already went up and patched spots with a fibered roof cement, but now that it is getting decently warm I want to coat the whole thing to seal it all off. What particular kind of coating do I use? I read somewhere that the roof should be "sealed" every couple of years. Well this have never been sealed.