I agree totally with Simon's method for depicting the nuts on wagon ironwork. In 7mm scale an argument can be made for using Grandt Line mouldings, but they are overkill for 4mm scale. The actual work of applying 10thou squares of styrene needs good light, good eyesight, and patience! Don't try doing more than a few dozen at first otherwise what I usually refer to as "doing a nut job" becomes all too literal.
BTW, though the GWR and others used rivets on their metal underframes, only the LNWR, as far as I know, used them to hold its wagon corner plates together. If you want rivets, particularly on a surface where you can't push through from the rear, then the Archer products are good. I might try to dig out some photos... but don't hold your breath.