As per the title, I work at a conservatory manufacturer and as such we often have large bits of hardwood (up to around 56x220mm section) which need to be joined with a mitre, such that the larger face is sitting on brickwork.
Typically we cut the mitres accurately, machine worktop bolts into the undersides and sometimes dowels or biscuits for locating the pieces together correctly on site. We seal the end-grain of the pieces with a purpose-made end-grain sealer, then paint with water-based microporous paint.
Whenever I go out to site to a job which has been installed for ~6 months or more, the sills and any other large mitred joins have typically opened up. I don't think that the pieces of timber overall have moved, I think it's more to do with across-grain shrinkage of the timber(?). I also see the same on pieces which other people have supplied, installed and finished such as internal window boards/sills.
Is there some way of joining them, some kind of sealant I can use etc. which will stop this from happening?
Note I have seen this question: End grain mitre joints for outside but I believe my question is a bit different because it's specifically dealing with very large section sizes.