Hi Karla, Please appreciate that I am going through exactly what you are! The wet plaster "problem" - same as me - I was worried it would set up before the pigment sunk in. In hindsight, a stylised sun, to go fulltime in a wet garden, took me about hour and half to paint a 16" x 16" panel, I had a lot more time then I ever expected, even with white cement in the intonaco. I have given it to my sister for a weathering test at the core of the rain forest - eastern Vancouver Island, BC.
For a 4ft x 6ft piece, to be painted and sold as a portable, this raises a number of issues. Firstly, at about 175 lbs (7-8 lbs a sq ft) you better start doing hard time at your local gym if you expect to handle it yourself! Secondly, you will have to fabricate a custom painting easel - perhaps based on a sculpting stand or one of those Black & Decker "Workmates". Thirdly, make sure you construct a sturdy packing crate, so it might be shipped in one piece - make sure the customer pays for it in the pricing. Truck freight is quite cheap, but if it is not packed in a precise way it cannot be covered by insurance - beware of this fact.
All for now. I was thinking of sculpting some Sri Lanka temple gaurdstones - do you know anybody interested? Evil persons who come to your door are turned to archeological goo by an ancient hindu curse - yikes but useful.
