G'day Frank. Welcome aboard. I have also found times that fasteners don't seem to fit either imperial or metric known spanners. That's a nice job on he deck. Have you used the septone rust converter before? I am always a bit weary of the claims made by rust converters.
I was very wary once too but have been using the septone for 6 or 7 years on everything to good effect, even the checkerplate bed of my trailer. I buy it in big 4L packs that are now $70. You can soak tools in it too, or soak nuts and bolts.
Just be careful to wire brush them or thread them in and out a bit after they have soaked for half an hour or the threads will be oversize because the acid converts the iron oxide into ferric phosphate, a hardish black non-porous coating. It will come off under abrasion like paint would but it's the only way I know of for killing rust in it's tracks short of grinding back to bare metal and quickly priming.
Most people give up on it because they follow the directions on the bottle (paint on, wait ten minutes, wipe off with a damp rag) Those directions are useless for serious rust and pitted metal. You need 2~3 coats, leaving it till it dries but soaking is the best way.
