back boxes

I'm just making holes for back boxes in my kitchen. It's exposed a lot of bare brick obviously. The previous backbox was totally rusty. What would you do to seal the brick to protect the box? Someone mentioned putting something called SBR on it, I've never used it, would it do much? Any...

I've run some cables up thru some back boxes for 240v sockets and was just putting some pvc channel over the cables (pairs - it's a ring) when I thought that it might be easier to get the grey outer off before the cables are fixed in place behind the pvc channel and up thru the grommets. Is it...