I paint my IRJs with brown stripes where they lie above sleepers to help hide them.
It works for me.
Bob

I'll agree with that. I can push wagons over IRJs no problem.

@Newportnobby must be a huge gap and/or well out of alignment if it causes derailments.   I'd be looking to lift and replace the track (with IRJs fitted).

Locos bridge the gap (including 0-6-0s) but there is the odd random derailment of 4 wheeled wagons when being pushed across the gap and, as it is intended to show this layout, that's a no-no in my book.

Putting miliput aside for the moment, do you actually need to fill the gap? I have isolated sections without using any joiners where the track is held by a sleeper close to the end so not likely to move. Confession, this was years ago when I was skint with the option of retrofitting a joiner if it became necessary. The layout in question survived over 20 years and I cannot recall any issues.

Link here: https://forum.mrhmag.com/post/mrh-202608-august-2026-13877155

Jon

I gave up using milliput for most things the moment my stepdaughter introduced me to good quality polymer modelling clay. With that I can make much better things and I've done stuff like a OO gauge model of our cat with it just fine, no way milliput would have worked.

My preferred multi purpose glue. / filler is glue'n'glaze   Will stay flexible in your application

Have to say, I've never even considered using filler to fill in a rail gap.
On the occasions where I haven't used IRJs, if I've been concerned about rails moving (through expansion, running or track cleaning) and making contact, I've just glued a sliver of plasticard in the gap.

Shouldn't be necessary to handle it.  When I've used it (and similar repair putty stuff from Lidl which is much better value)  I simply slice a tiny amount off with a scalpel and manipulate it into place using the blade. 

To be honest though, I'd persevere with the isolating joiners which will maintain proper rail alignment.  I've never had a problem with them.