What's a good way of roughening the under surface of tilt-up slabs to take a plaster finish?
A cheap and easy way is to spread a layer of coarse, wet sand over the bottom of the form, roughening the surface uniformly before placing begins. After lifting, the sand can then be brushed off to give an irregular finish offering a good key. If for some reason this isn't practicable, you can place the concrete over a thin ribbed or grained rubber mat laid in the form. These can easily be stripped off for re-use, but the working face of the rubber should be oiled.