I am sure, all this is not really a secret for you
But I'll just describe how I did it with my sewer:
Seal the base's / diorama's ground well, first. I used at least 2 (if I remember correct even 3) layers of thinned Enamel / Email matt varnish as ground sealing. Didn't trust acrylic varnishs there...
Then, don't forget other parts of the diorama which
look like they
seem to form a "natural" barrier for the resin, but might be a little permeable, e.g. (plaster) walls and the like on one or more sides of the basin. In my case, the walls are made of plaster, and there even are a couple of "larger" gaps and holes of 1-2mm diameter between the bricks. Additional to the varnish treatment, there has been no other chance to seal them than to use silicone for these gaps, one by one, as well.
Then, as Alexandra mentioned, (2mm) acrylic sheet for the side limits which I "glued" to the diorama already applying thin lines of silicone. Sealed all joints between diorama and acrylic sheets from the inside with a little silicone all around, using toothpick and screwdriver. Payed attention to really push the silicone into every gap and hole. Last, sealed all joints from the outside with (much) more silicone, now everything double and triple protected.
Well, finally controlled everything once again and again.
And I remember very well how nervous I've been when I poured the first resin filling into my sewer... :sweat:
Anyway, the process of sealing can become quite difficult, if the basin doesn't have a simple geometry. E.g. comparatively stiff acrylic sheet of 2mm thickness can only be used in case of straight borders, of course.
Perhaps it's also a question of the specific base / diorama or rather the way how it is built up. E.g. my sewer's modeled plaster ground is max. only 1mm thick - beneath, there's directly a solid hard wood fibre board, 3mm thick, which might have prevented any seepage even if the Enamel / Email coating on the coloured plaster didn't work in reality as expected.
Things may end up different if there's (only) (high) permeable material underneath the basin, which cannot be sealed easily, e.g. if it absorbs any varnish without effect...
Finally, after I did my first experiences with resin now, I can say I don't hate it. But of course it will always be and stay something I'll face with some attention and care... Who knows whether it will work always that good...?
