Have just had a mate mig the holes closed, I now am going to bog over the mig welding 

Are you getting someone else to paint the guard?.
If so you might as well let them do the bog work as they will have done it before. Also they would probably prefer to do it themself if they are worth their salt. It is only a small job looking at that guard.
If you are going to paint it yourself then using bog is pretty easy. You basically mix it up to the 50/1 ratio or what ever the product says on the can, lay it on with a putty knife or even a butter knife so that it sits proud of the eventual shape you are after and when it sets sand it back till it is the right shape using finer grades of sandpaper as you go. Using a cork block to hold the sandpaper is good practice too.

I'd let the painting pro do it in that case mate.
Be nothing worse than you bogging it, painter paints it and then has you to blame for some imperfection on the final job.

Agree with FC bloodog. An accident, then repaired by a good mate, told me not to touch anything but let him bring back to standard, if not better. He would say "do what you're good at, and I'll do the same". He was one of those magicians that could panel beat, spray paint and motor build, V8 race circuit, amazing bloke.I know I love doing shit in my shed like that too, but end result is what comes mind.