Didn't know that you could use Oxalic on gelcoat ( although it works well on stained wood)usually comes in crystals that need desolving in water and takes a while to work is then washed off with clean water./forums/images/icons/wink.gif
It comes in powder form, I mix a teaspoonful in a pint of water,
You can make a stronger mix if the staining is heavy, A little goes a long way,
so there is no need to make up a large amount.
I use a paint brush to apply it , care should be taken as it does burn your skin.Brush it on and watch the stains dissapear,
If the stains will not shift mix wallpaper paste with it until you have a thick mix then apply
and leave,
Just rinse off with fresh water, job done!