Looking good+large

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Cleaning the surfaces is important........1200 W&D paper will polish the surface, however it does leave a fine dust deposit which does need to be cleaned......Isopropyl 99.8% alcohol will remove this
An alternate to the W&D paper is non soaped steel wool

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Having said this, you have quite a
mass of brass that will require a general
preheat over a refractory brick surface before to the actual or second torch heat for the silver soldering process
This preheat [complete with the fluxed surfaces] will tend to distort the plate surfaces + the reinforcement bars
From this, you could consider pinning the reinforcement bars with say 1mm brass wire as retaining rivets to stop distortion

of the reinforcement bars
Again, you should research the actual S/S + fluxing process prior to commencement......
cupalloys.co.uk have some great on line tutorials......well worth reading

Derek