I agree with ZombieEng., you're probably using too much water.  I find that most plasters I've used require anywhere from 28 to 33 parts water, to 100 plaster, by weight.  Any more water than that and you have excess that floats to the surface and causes "short" blocks.  Any less and you need a lot of mechanical vibration to get the plaster to flow properly.  A scale can really be beneficial to get your ratios figured out the first few times; after that you can probably start guaging it by eye or feel.

The other way of doing it consistently is to mix it 1:1 by LIQUID volume, not by the volume of the dry powder.  To do so, mark a cup with a line below the half-way point and fill it to that line with water.  Dump the water temporarily into a second container and re-fill the cup back up to the line.  Pour the first quantity of water back in and you now have twice the starting amount.  Mark a second line on the cup at the new level and you've got the perfect measuring device.  To mix a batch of plaster, fill the cup with water to the first line, then slowly sift and stir in plaster until it reaches the second line.  It's easy once you set it up, and VERY consistent.  Some specific plasters need to be mixed a bit thicker than this, some a bit thinner, but it's a reasonable starting point that works for most, and you can easily adjust a couple of percent one way or the other by sight.

Ideally, you want it as thin as it can get without leaving water on the surface when it settles.  The only time I use paper towels in the process is for cleanup afterwards.

Last Edited By: Kovax Oct 25 11 4:37 AM. Edited 1 times.