If you grind them down, you grind them away. You are going to have to make them lower than the surrounding material for paint prep as you know. Not seeing a pic I suspect I would try to sand on the welds-then maybe poilsh/buff them as smooth as possible. If you are a good sheet metal worker you can tap them slightly low and use an appropriate filler. If you leave them raised, you would have to cover the whole side with a filler. Being in the US, I could not advise what type of filler, as I know not what is available to you.
