help! sanding epoxy in the sun

finally have time to sand and need to do it outdoors.

1) Its 90 degrees today and the boat will be in the sun. will it melt or soften the epoxy so that i cant sand or do damage to epoxy?

2) Also with initial sanding using 80 grit should the hull already be a uniform gray? Do i keep using 80 grit till it is uniform or switch to 120 etc till it gets that uniform grayness? is it critical for it to all be uniform?

thanks!

michael