The thing to remember with Piko/Pola kits is there is a colour paint wash over all the moldings to give them “highlights” and/or a weathered appearance. I haven’t come across one of their kits that doesn’t have this feature. Another German make you may come across, Vollmer, does not apply a colour wash.
It is important to remove this “wash” coat from the mating surfaces of all joints on the structure, use sanding stick/block c 600-400 grit, or it will impede the effectiveness of any solvent you are using to bond surfaces together. I also subscribe to the idea of using silicon caulking to strengthen joints. Another method would be to use two pack car body filler, particularly with quick cure resin structures, as a strengthening medium. I have had buildings out 365 days of the year for up to 12 years, temp’s from -15C to +30C, in full sun or shade without the need to rebuild, with just the use of solvent based products - so long as they are properly prepp’ed before assembly.