Anyone know what thickness of steel is removed by phosphating (keyphos?) or blast cleaning?

Cough! electrolysis..

what you cleaning?

grit blasting will remove about 0.001-0.002" if your just using it to clean or remove paint. Using large grit and turning the blast pressure up will remove more.

If your just using a small spot blaster at home and not a large industrial machine then i wouldn't worry about it too much.

However if your are blasting anything thinner than 1mm I wouldn't reccomend blasting as it may deform your workpiece if your not careful.

best to mention that a lot of heat is generated on thin sheet from the friction of the beads and this can distort.