Both Abralon and Siaair Velvet are both graded using the European or FEPA standard. All grit levels should be indicated with "P" before the number and the grits should be indentical. They both use the same abrasive that is on wet/dry sandpaper: silicon carbide.
Both are backed by a foam that absorbs water and conforms to a ball''s surface easily with your hand as a backing.
For whatever reason the Siaair seems to me to last a little longer and they are slightly cheaper.
I got mine from a woodworking company called Grizzly. They sell them in 5" and 6" diameters. For the additional money, the 6" ones seema better value. Abralon comes in only 6" sizes.
http://www.grizzly.com/products/searchresults.aspx?q=sia%20air5" ones are $2.95 each. 6" ones ar $3.50 each. plus shipping of course.
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"None are so blind as those who will not see." Edited on 10/4/2009 6:37 AM