I've used backerboard for outside tables and never sealed anything but the grout. The rest can be painted or left as is.

It doesn't resemble sheet rock much. It's solid and harder.

If you can, buy a scrap piece for less money than a large sheet. I got a piece that was leftover at a hardware store for very little money.

It's concrete based, so be careful in cutting it - wear a mask. I LOVE it to work on even inside stuff. You can paint it, chip the edges and faux marbelize it - I did that for a collage. It comes in 4'X8' as does sheetrock and plywood, but you can get smaller pieces at Lowes. You can score it several times w/a box knife and then it breaks. Here's the pic of the collage. Granted, I need lessons in faux marbelizing!!!

Slow what a cool collage. I wasn't sure what you meant by chip the edges but now I understand. I've only used it once and also found it real easy to use. I think I'll chip the edges on mine though. Slow did you hit at the edges with a hammer to chip them ?
Cindy

I just used my fingers on the collage, but used a tile nipper on the frog - much easier.