I was watching the cake decorating at a local grocery store and the decorator was dipping her spatula into hot water and without wiping it off she smoothed out the buttercream...I just tried it on a small cake and it works perfect..the woman did shake off excess water...I just tried it with white...1/2 butter 1/2 shortening buttercream

What type of BC are you using? Usually the water causes the buttercream to get sticky or worse weep color. If I don't dry my knife well enough, I get little pools of water along the edge that can cause issues with color etc. Glad it worked out for you