I do things a little differently and I I would have no problem with doing what Tony is after and in a very short time. I used to spray paint my backer boards and then I usually found that the edges would not always get covered very well and I really did not like that.. Now, I never use rattle can paint on my backer boards but rather, Now I buy the blue mechanic's paper towel and cut each sheet in quarters. then i fold one of the quartered sheets again in half, twice, in effect I quarter it again. Now I dip a corner of it lightly into a pint size can of black paint and rub the edge of my backer board with the dipped rag. once the edges are done, I lay the backer baord on a rag and continue to cover it with my rag dipping it as needed. I rub the paint into the surface as if i was polishing it so that the paint is quite thin and even. This entire process takes about 2 minutes and dries quite quickly. it also is covered very evenly every where,including the edges. So it is cheeper, faster and i do not miss any places nor do i need to put on a second coat..Give it a try you may be pleasantly surprised. My last int lasted over a year and at the end it had little dried paint floating around that was not a problem even with that
Dick
heppnerguy