Best filler for newel post joint?

5 Apr 2014
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United Kingdom
I added these steel plates, 4 stainless screws each to stabilise flexing in the upstairs newel post - Top post was is a reproduction joined to original base but I didn't have that much original to join onto.

Merrily I routed the joint fixed the screws and then realised I don't know the best filler for such a large gap. It needs to be strong paintable (will never be stripped) but flexible because there is still some movement in the joint. Routed section is about 16mm wide. Resin? Car Body?

Maybe I'll should expect the join to crack along the line, then add flexible caulk after?

Any ideas?
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Isopon flexible (elastic) car body filler......two pack, quick to set and sands beautifully in 20 minutes.
You could be surprised how much this stuff can flex!
John :)
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Thanks for advice guys. I used a two part called FILLTITE. just like car body filler I used on my renault 5 10 years ago, but as geraldthehamster suggested probably was easier to sand. I used some mesh behind to strengthen, left over from fixing the Renault. Much easier to do than I expected and no matter how much I 'Jer-Jer-Jiggle it Granville' it's not cracking!

Well pleased with the result.
Thanks again :)

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