What do you recommend (or sell) for doing joints where I need to adhere two pieces of Foamular together to like wrap around corners? I hate seams and I want to do this right the first time instead of having to go back and fix it or redo it.

Along any seams and corners, you should use Mesh with whichever foam coating you choose. 

The whole room/game is about 800sqft with multiple hidden rooms. All the walls, tree and statues are being coated. Do I need to reinforce the foam coat?

I usually suggest making the first coat a thin scratch coat with Boost, then getting strength from putting on a thick coating of straight foam coat. Dollar for dollar he will get more strength by doing this. If he needs a thinner coat he could probably go 25% thinner with Boost and get the same strength.

It's a good idea to at least use Mesh on any protruding corners and along seams. I made some planter boxes with and without Medium Mesh on the faces and was amazed at how much strength the Mesh added.

Considering using your products for my theming business. Was wondering if you sell the mesh you show in your videos. Thank you for your time, Richie Cash/Theme Weaver Design

Yes, we sell two grades of Mesh. I have found that with the Medium Mesh it is best to start with a fairly wet foam coat mix so it penetrates the Mesh. If you don't mind the added expense of Boost, it works super well to make an initial thin wet bonding coat with Boost added. Then you can apply the next coat(s) without Boost, and as thick and dry as you want.

Great customer service, fast shipping, excellent product. Building a small 8 ft boat . Will be purchasing Styroplast when ready.