I have a small firebox with a couple of dozen side stays in total and have soldered them by having the ring of solder inside the waterspace with heat applied inside the firebox, as described by Julian here.
As it is such a small space and the door plate had already been fixed (as per LBSC instructions), I had to use a cyclone burner as the standard one wouldn't hold a flame. Several of the stays didn't take the solder the first time but now all seem fixed tight. I used a ring of dia 1mm Easyflo in 1/8" I.D. rings. Each of the solder rings were pushed against the face of the (inner) firebox wrapper sheet, so I am fairly certain that if the solder did take, it went to the underside of the stay rivet head. My thought is, do I need to now coat around the heads of the rivets with the same solder to ensure steam tightness? In some cases I cannot see a clear ring of solder under the rivet head.
The same query applies to outside the outer wrapper. It is riveted to the throatplate and the crown girder stays are fixed with P.B. nuts and bolts; The parts were all silver soldered together but the solder has not appeared on the on the outer wrapper surface. Is a coating of soft solder warranted?
for the outer wrapper side of the stays the silver solder is applied when the job is up to heat running a rod of silver solder over the stays (shank of rivet protruding), same as you would normally do other boiler joints. any that dont show a fillet of silver solder need doing again. it is quite easy to miss a few, so not unusual and nothing to worry about. however you need to flux over all joints that might be affected by the re-heat.
the above method isnt practicable inside the firebox hence the ring method hard up against the inner firebox inside the water space. if no 'ring' of silver solder appears on the inside of some stay heads, then you very carefully need to reheat the inner firebox sheilding the unaffected areas, and silver solder the individual stay heads that havent 'taken'. this isnt so bad as it sounds so long as you draw out a sketch you can see when doing the re-heat of the problem stays for reference as you cant see which are unaffected otherwise in a flame roaring away. watch the flux and that will tell you when to apply the silver solder momentarily taking the flame away so the flame doesnt melt the silver solder before it gets to the area affected. you will need to flux everything in the inner firebox for such a job.
you cant rely on soft solder (comsol) to caulk stays not threaded in the plates and nutted inside the firebox. if of the unthreaded rivet type they MUST be silver soldered.