Here is a car made from plaster. The finish ( and colour ) need more work, however the idea of using a mould made by splitting an MDF model seems to work. The top and bottom of the model were moulded separately and the bonded together with plaster. It was shaped, smoothed, primed and then spray painted. This was a test model to check the viability of the process. The wheel details need to be developed in particular - I guess the wheels should be recessed which would make them look more realistic. Wind mirrors etc need adding.

Close up detail reveals the imperfections of the plaster. Care is needed here to get the surface smooth BEFORE applying paint.

Here, on the underside view it is possible to see the bare plaster. I have also scraped away the cill area to expose the plaster. The model is scaled at about 1:18.