Many of the resin castings we produce are made in open-faced moulds which can leave a meniscus or a convex undersurface. This is easily dealt with by use of a heavy grit (80 or 120) wet-and-dry paper. Resin dust is toxic and so all sanding should be done whilst wearing a dust mask and in a well-ventilated area. Even better, use water to wet down the sanding area and casting as this will almost eliminate airborne dust; we have omitted water in these photographs for ease of viewing. Any water/dust residue can be easily cleaned off - see Resin Paint Prep tutorial.
Place the casting evenly and squarely onto a flat sheet of wet-and-dry, fixed onto a flat, hard surface. Applying even pressure, make repeated circular movements with the resin piece for 5-6 passes, the reverse the direction of travel for a similar number of passes. If necessary, rotate the piece through 180 degrees relative to the sandpaper and repeat.
Finished underside of cleaned casting showing flattened/levelled area. This is a light sanding taking only 5-10 seconds and removing very little of the detailed resin piece.
Some of our larger models are cast with a hollow void underneath to help reduce postage costs and overall weight. The flash that results can be removed in exactly the same way as described above.
Again, repeatedly pass the bottom-down casting against the sheet of wet-and-dry in a circular pattern, clockwise and then anti-clockwise for approx. half a dozen rotations in each direction. This should be sufficient to remove flash and level off the model's underside so that it sits flat on your table.
Completed, levelled model underside. Now refer to Resin - Paint Prep for details on cleaning off sanding residue.