Rich, You can use whitewash or for cold weather areas we use grease thinned out, applied with either a roller or brush. A note of caution, we have a 1/4" thick steel plate. If I remember right a few years ago I tried doing POI work on it. First shot sent lead right back at me. Fortunately I had glasses on. We also have a wooden frame to tack paper to for pattern work. Good luck, Bob
idoc, it depends on what you want to use the board for. If you are looking for a grease board, solid steel plating works well. By preference is for thinned white lithium grease. I like a roller to even it out, but applying with a brush and pail makes logistics easier. You can always roller it out after application anyway.
If the board is to be used for POI testing or real patterning, a metal frame with heavy metal mesh behind is better. The mesh lets the pellets pass through the paper or plastic you are using to record the results instead of bouncing back and ruining the pattern. This is especially true for ranges closer than 40 yards with a full choke.