Frosting is a sign of to high a temperature of either the lead or mould , I find it is usually the lead temperature . Try a lower temperature the lino type seems to have an element which dosen't like the temperature.
Having said all of the above frosting is nothing to worry about ,it is only cosmetic.
This explains a lot about lead mixes
http://www.lasc.us/CastBulletNotes.htm
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