wataru51,
WELCOME TO THE ORIGINAL AWS WELDING FORUM!!
Now, a couple of corrections to the wording of your question:
1) Leave the word 'simple' out of it especially in quotations where you are attempting to specify the weld type involved.
2) There is no such thing as a "Butt Weld". It is a butt joint which requires a groove weld of one geometry or another to complete.
3) "Flat" is in the wrong location as a descriptor of what is being required.
Thus, if I have understood correctly, the sentence would read, 'When you have a butt joint with a welding symbol calling out a V groove weld from one side with a flat finish symbol'? Correct?
If that is the correct interpretation of your question, then the reinforcement of the V groove weld must be finished flat. If they have not specified the method you may choose how to accomplish it; G= grinding, H= hammering, M= machining, C= chipping, etc.
Now, anything else is dependent upon the balance of the welding symbol. If, FOR EXAMPLE, the same symbol is on both sides then it means you need to do the same from both sides. If not, then it is only the side the symbol designates; V symbol on bottom of reference line= arrow side, on top of line= opposite side. Only the side that has any directions requires any work. No matter how bad or good the weld protruding from the root side may look. Unless it is just a company QC thing for cosmetic appearance.
He Is In Control, Have a Great Day, Brent