Mad Mike
Well-known member
The problem you have with Vinyl is expansion and contraction, everything has to be slip-jointed otherwise you'll end up with a twist, warp, bow, and sometimes cracks. If you don't like the look of a return, vinyl j-block fitting and your dowel/lag idea might work. These trim blocks are designed for penetrating hose bibs and conduit, which might make the grades for the estheticsand fnnction.At this point with that railing I would either fabricate and L bracket to fasten to wall on outside where I didn’t have to look at it , or as Greyghost said drill through and lag bolt into house using piece of black ABS as a spacer .
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This type of trim can stay rigid and be caulked while allowing the siding around the block to slip and move.