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Deleted member 174758
Rebar is very soft steel and I reckon it will bend easily if subjected to side pressure. It's also a bit on the small side (8 to 10mm?) to secure what looks like an 84mm (3-1/2in) newel post. I've fitted bigger newels in hotels where the manufacturer supplied a stainless steel rod to go up the middle, but they were about 30mm diameter and something like 450mm long and we installed them using a thin smear of epoxy resin they were a close fit), not car body filler (which isn't a structural filler and probably won't take a lot of side loading which is what you will get on newel posts as folk do swing on them as they round the corner at the top of the stairs). I have to agree with Norcon that it's probably a bodge too far - and the OP's trying to avoid that.I've done it loads of times, use it all the while. Sets that solid you'd never know it wasn't a tight dowel.