Many thanks for the suggestions. It seems it's much as I thought: I am going to have to paint the seams, rather than tape them; I think I'll try the silicon sealant + white spirit option and see what happens. Hopefully, it won't rain and the b****y stuff will dry properly so it isn't tacky!
It dries very quickly as it happens, mix more than you think you will need,if it gets gloopy you can discard it, it is so cheap to make it does not matter.
I use a plastic cup like the ones that are used in soup or tea and coffee dispenser machines, good source for a free cup.
Be generous with the silicone sealant, and buy the cheap stuff (screwfix and toolstation have low cost silicone), not the type with added this or that enhancers as additives can be counterproductive (you need to google that for more in depth info).
My final mixed batch is approx 2/3rds of a cup, about 1/3rd of the cup is silicone,it is easy to add more white spirit but you have to mix very very thoroughly.
1/2" or 3/8" paintbrush is ideal and it paints on with surprising ease and it is easy to keep a consistent line width, actually very nice and quick to work with, I have found the finished results very pleasing, in my case(s) I have to look rather closely to see the painted silicone seal, it is unobtrusive.
