The way I see it it all comes down to why the solder joint failed. If the joint failed due to corrosion caused by hot blue,acid flux or moisture trapped between the ribs then a full relay may be required. If a small section comes loose and no corrosion is present (both surfaces appear tinned ) a spot repair is appropriate.
If you feel the need to put something between the ribs I would use liquid rosin flux . It is something that belongs there anyway, it will dry and leave a protective film. If a repair is needed down the road you won't have to worry about removing something that may contaminate a solder joint.
Mark
http://www.mbabllc.com
