Ice can distort the flow of air over the wing, diminish- ing the wing's maximum lift, reducing the angle of attack for maximum lift, adversely affecting airplane handling qualities, and significantly increasing drag. Wind tunnel and flight tests have shown that frost, snow, and ice accumulations (on the leading edge or upper surface of the wing) no thicker or rougher than
a piece of coarse sandpaper can reduce lift by 30 percent and increase drag up to 40 percent. Larger accretions can reduce lift even more and can increase drag by 80 percent or more. Even aircraft equipped for flight into icing conditions are signifi- cantly affected by ice accumulation on the unpro- tected areas. A NASA study (NASA TM83564) showed that close to 30 percent of the total drag associated with an ice encounter remained after all the protected surfaces were cleared. Nonprotected surfaces may include antennas, flap hinges, control horns, fuselage frontal area, windshield wipers, wing struts, fixed landing gear, etc.
Some unwary pilots have, unfortunately, been caught by surprise with a heavy coating of ice and no plan
So there you go Frost no thicker or coarser than a piece of course Sandpaper can reduce lift by 30% and drag by 40% so don't risk it
Full article worth reading linked below
http://flighttraining.aopa.org/pdfs/..._Icing.pdfPace
Pace