Jeff Layman wrote:

Anyone got any experience of using this sort of product to fill small
cracks in PVC doors?
http://www.repairingproducts.co.uk/product/cosmofen-rm-pvc-gap-filler/

Not that, but I have used this

https://www.platinum-supplies.com/everflex-165-small-gap-sealer-pvc-adhesive-plastic-everbuild-white-sealant.html

In particular, has anyone used only part of the product and the rest
sometime later? There seem to be several mentions in reviews of having
to use it all at once.

I still have a part tube in the garage, but I expect it's gone off ...