Sonic checkers

OK just got back from IL and I cut out two pieces of cylinder walls from a 360. I calibrated the checker and the velocity came out to be 4360. I still have some more checking but that velocity was reading the walls within .010 or less. I checked 5 blocks out and was surprise to see how steady and consistent it read. The pieces I cut out had some nice white powdery build up on them. I cleaned it off and mic'ed it then calibrated the checker....I then measured some places without cleaning it then wrote the number down and marked the spot. After cleaning the whole piece I went back and mic'ed it in those spots and the white crud didn't effect the reading. Neither pieces had very much rust so I'm not sure what rust will do to the reading. Also the temp didn't make a difference. I warmed up the block to well above freezing then checked them and the next day I let them get below freezing and it still read very very close as to when the blocks were warm. I'll post more info after I do more testing.