Painting the patio railing required removing the short section on the garage, which stalled with a thoroughly galled / corroded nut on the 2 inch bolt going through the wall. Deploying a Dremel slitting wheel and bashing the slit open with a cold chisel saved the day, as shown in this staged reenactment:
It seems square head bolts have gone out of fashion, at least in the 3/8-16 size seen here, over the last half century:
I reused the lag screw with no qualms at all.
The local fastener emporium had square bolts ranging upward from 3/4-10, which wasn’t much help. Amazon has ’em, if you spend enough time rummaging around in the debris from its search engine, at a buck apiece in lots of ten. Fortunately, a local big-box home repair store had 3/8-16 hex head steel bolts and square nuts, so I needn’t start from scratch.
Start by turning off the hex head:
Thread the end, starting in the lathe and ending with a die turned just barely enough to accept the nut:
Epoxy the nut in place and sand it to rough up the surface finish enough to hold the primer:
Yeah, that’s a nasty little zit. Fortunately, nobody will ever notice.
Prime & paint the railing, affix it to the garage wall, then prime & paint the bolt:
Thing looks like it grew there; tell nobody about the zit.
The yellow blotches decorating the shiny black paint come from the pine trees across the driveway. The first day of pine pollen season corresponded to the second day I intended to paint; the dust clouds were a wonder to behold.
Bonus Quality Shop Time!
The far end of the railing around the patio has a bracket against the house siding with a hole intended for a 1/4 inch bolt they never installed, perhaps because there’s no way to maneuver a bolt into the space available.
The threads on the 3/8-16 bolt may be wrecked, but turning the shank down to 1/4 inch isn’t any big deal:
Part off the head with a stub just long enough to fit into the bracket, epoxy that sucker into the hole, and paint it black:
The square post on the left goes down to an anchor in the concrete patio, the railing is welded to a 4 inch column a foot away, and the end of the railing isn’t going anywhere; the fake bolt is purely for show.
And, yes, the dust atop the railing is more pollen from the pine trees responsible for the weird green-yellow reflections on the vertical surfaces.
No boomstick required!
The Smell of Molten Projects in the Morning 
Maybe I missed something…. why not just replace with a hex-head bolt and nut and be done with it?
It’s a stylin’ thing: all the other bolts have square heads, so a hex would be ugg-yoo-lee!
I decided a slightly larger square head would be OK; I’m not completely, ah, nuts.