soldier paste should not be used for re laying ribs, its not strong enough, and is highly corrosive, its also near impossible to get the joint water tight. As a professional gunmsmith ive seen all sorts come through my workshop and the best advice is if your not confident your doing the right job, send it to someone who knows.
Spot repairs to ribs almost always end in failure eventually and should only be considered a temporary fix, the reason being that without lifting the ribs entirely its impossible to ascertain the quality of the two mating surfaces, the reason the condition of the surfaces matters comes down to a simple basic fact of soldier and braze joins, that the tighter the joint, ( ie the smaller the gap between the parts to be brazed or soldiered ) the stronger the joint. This is why best gun makers put many many hours of hand work into fitting the ribs to the curved surfaces of the barrels.
So, how do you re lay ribs; in essence its simple, first you heat up and remove the old ribs carefully.
Next clean off the old ribs and mating surfaces. To quickly get back to bare steel with minimal damage to the surface I have found using a brass wheel on the bench grinder a quick way to abrade the old soldier away.
Clean and de grease the old ribs and mating surfaces again, and apply masking tape either side of the mating surface on the join, (so the only steel you see is where the rib connects ) then apply soldier paste ( i use fryolux ) to the mating surfaces, its sometimes good to thin it out a bit with meths.
Using the gas torch heat each rib component and barrels up in turn and as the solider in the paste melts wipe it over with a wet cloth to tin the surfaces, do this all over but as far as is possible try to keep the tinned area only where it needs to be for the joint.
Next thoroughly clean absolutely everything, scrub it to high hell with soap and cold water even throw a bit of bicarb on a scrub it with that and then scrub it all again with cold water, the goal is to thoroughly neutralise any residues from the highly corrosive soldier pastes (*I have seen first hand this residue eat through barrels in as little as four years)
once everything is clean and dry you can wire everything up your wires must not too tight or too loose, just holding everything just so, old horse shoe nails are handy wedges for this task but you can make some simple wedges out of 3mm steel. Check and check and check the wiring.
To soldier use i big flame to heat the barrels evenly, do not focus on one spot too long, i use the type of torch that is designed for burning weeds, as it has a big flame that's plenty controllable and i don't have to hold a trigger to keep it lit you just set the gas flow and set to.
Use raw powdered rosin as the flux, it smells good and its not corrosive.
The solid bar soldier Holland and Holland use in their promotional videos is just bars of pure tin you should always use pure tin to re lay ribs. Its lighter and stronger and something about the lead component in other soldier also promotes corrosion so pure tin only is what you should use, you often have to buy this in at ingots and melt it down into handy bars for rib work.
Once finished you should clean everything off using a brass chisel to remove the excess rosin and tin ( save the tin and re use ) Once completely cleaned off submerge the barrels totally in water, if there are any bubbles, mark the gap areas and apply more flux and tin until all problem areas are eliminated the reason being is these leaking areas cause so many issues for barrel finishers.
the thing i hate most about strip and relays is it takes a full day by the time you have struck up the barrels again and its a smelly dirty messy job, and you get blobs of molten rosin and tin on the floor, but its never really as much of a chew as i think it is.