I'm building a diorama to give as a gift to a very good friend. It is a garage / maintenance shop building. It will be finished on 3 sides with the back fully open. The floor is a sheet of polystyrene 24" L X 12" W X .030" . It will be mounted to a thin board cut to the exact size as the floor and then that will be attached to a larger board to allow a couple of inches of landscaping across the front and sides.

The floor piece has been primered with red automotive primer and 3 coats of Woodland Scenics Road Systems Top Coat in concrete have been applied. It looks really good!

The items I am putting in the shop are made of wood, cast metal, the brown/beige resin, the softer gray resin, polystyrene plastic, cardboard, and vinyl which is the tires of the two vehicles I am using in the scene as well.

I am leery of gluing anything to the coating directly. Some of the detail stuff like the cast metal upright air compressor are tall and a bit top heavy. My concern is things falling over or moving from even gentle handling and tearing the finish away from the floor.

Should I overcoat the concrete finish with something or will it be OK as it is? If I should over coat it, what should I use?

Lastly, I had planned on using a good brand of two-part epoxy as the adhesive to hold everything down. Is that a good choice or will some of the materials I listed be better served with a different adhesive or adhesives? Are there any specific brand name products that come highly recommended for this application?

Any advice, experiences with doing something like this, words of wisdom, etc would be and are greatly appreciated. I want this to turn out well.

If you are happy with the look of the top coat, I would be hesitant to put anything else on it as generally another coat will change the present appearance somewhat.
I am not familiar with this product so I can't directly comment on it but if you are worried about pieces (such as you compressor) not adhering strongly enough why don't you use a small piece of brass rod and drill into the bottom of the object to be attached then drill a small 'accepting' hole in your base where the object is to sit. Basically the same way most dios have their figures attached. It gives them more strength during the 'jarring' a diorama sometimes takes when being transported etc. Basically, pin them in place.

Cheers,
Charles

That's an excellent suggestion! Pinning things in places will take the stress off the side loading of the taller pieces. The smaller stuff doesn't have enough mass to be an issue. The problem may be with securing the vehicles since one is a wreck of sorts and the contact points with the surface will not be conducive to drilling and pinning anywhere else would be visible. But there again, neither vehicle model has much mass to it.

Again I can't state adequately enough how happy I am with the appearance of the concrete finish from this stuff! The color is right on the mark and the texture when it sets is remarkable. I also have some Woodland Scenics asphalt finish that I haven't used yet. If it's as good as the concrete is it will be stunningly realistic.