The key is to move back further so the sealant is almost dry when it hits the foam. Alternatively, you could use sealers designed for foam - like the Army Painter version. Or do what I do - coat it in PVA glue thinned with water.
I don't really do FB, but I have one I mostly ignore. Maybe I will look into this group.If you do Facebook - and I totally get it if you do not - I only use it for gaming stuff. There is a great group called Foam Modelling Workshop. A lot of amazing stuff show cased there.
My process is not too dissimilar, but a slightly different order to my steps.I actually cut most of these by hand because I don't have a table yet - but when I get one I will redo these or just make a set that fits together better. I think next time I will put some small washers between the layers to give them some heft as well.
As for dry brushing: black matte/mod podge layer first (two coats), heavy coverage dark gray (mixed with a little bit of regular gray to lighten it slightly), painted some of the "off-color" squares (after seeing that ziggurat!) and then dry brush regular gray for the edged and bevels, and then very light dry brush with "fog gray" over various places to bring up texture.
Next step is a black wash which always comes out too dark and dirty looking - so looking to once again adjust my mixture - not sure why it always looks so bad. Any advice for blackwashing terrain, @pogre?