The glass block wall can go over kerdi membrane, but only if the kerdi is on top of base substantial enough for the weight of the glass block. The glass block can not go on top of the foam schluter curb. Either frame a base for the glass block wall or form it out of deck mud. Mongo

I live in San Diego, home of the giant brush fire. I guess I don't need the drain that much, it just looked convenient and easy. What I am really interested in is finding out how well the concrete would have to be sealed.

I'm not a pro, I'm a stuggling [but enthusiastic] amateur. But as such, I've screwed up enough that I've developed some insights about how NOT to do it. I think it's helpful to use relatively small chunks of kerdi when you are first learning the material. One of the most important factors: get a lot of mud up quickly, comb it, get the excess back off. You don't want to diddle around filling in the thin spots. When you first comb, if the ridges aren't perfectly uniform you don't have enough mud. When laying a tile, it isn't an *awfully* big deal if your ridges aren't perfect. With kerdi, it is. Comb so that the ridges will run the opposite of the long direction of your sheet. If they run the same way, the air from the valleys will have a looong way to go to be expelled, and you'll have to fight it much harder. My first tub surround, I was too enthusiastic about running my hand over the sheet for the initial bed. I trapped way too much air. After a couple showers, I got to just bedding the top, and a line down the center to hold it in place before going to the blade. I found the quickest way to bed was to use fairly short strokes, diagonal to the ridges, top to bottom, center to edge, being careful to leave no air behind. The air at the lead of each stroke should always have a clear path to daylight. My first couple pieces, I was so slow getting it bedded that the mud towards the edges was past it's prime by the time I got there. I was too aggressive using the trowel for bedding, and ended up rubbing most of the fibers off a few areas. I had much better luck using a putty knife with a nice bendy blade. Use the flat, trail the edge. Judge your job by color: if it's uniform gray, you're well stuck down. Don't try lifting a corner to see, it won't be adhered until the thinset dries. Trust your instinct: when the thinset dries, the sheet gets lighter colored. If its uniformly gray when you first finish, don't second guess later. I found the banding was much easier in corners than trying to wrap the full thickness stuff around. If you are working alone, I think it's a good idea on a larger shower to have all the pieces pre-cut and labled as to location before you mix the thinset. Otherwise, the thinset in my bucket would start to go off before the job was done. I still found it best to mix batches no larger than 25 pounds at a time. It was also easier for me to wait overnight between the walls and the corners. When doing one right after the other, I had trouble with the stuff under the wall sheet still squooshing [technical term] around when I was trying to bed the corner bands, led to a couple pretty uneven spots. Hanging kerdi really developed my trowel skills. Hope I still have them, after a month off baths while finishing my kitchen-- one tub surround left to go...

If you have a 1/4" to 1/2" gap at the tub that needs to be filled, consider installing a veneer of cement board on the walls. That would cover the gap and give you a pristine surface to work off of. The negative to that will be twofold; you'll need to consider the detail at the edge of the tile on the two end walls with regards to hiding the edge of the cement board, and you'll want to make sure your valve trim kit can accommodate the extra thickness of the cement board. The cement board edge can be hidden with a tile trim piece, a return, or with a wood trim boarder that melds with the other trim in the bathroom. The semi-gloss paint on the walls is a big negative for me. Thinset is less likely to develop a bond with it, and because of it's permeability, semi-gloss can create difficulty with modified thinsets drying versus an unmodified curing. With that info given, I'll try to answer your questions: "1) Can I use a lightly modified thinset (versabond) instead of unmodified? (I know this will void the warranty, but will it hold better?" Modified will develop a stronger bond then unmodified in a pristine installation. But you could encounter drying difficulties by sandwiching modified thinset between the semigloss paint on the wall and the kerdi membrane. 2) The plaster wall substrate is not even, particularly where the wall meets the tub, as mentioned. Will this cause a problem when tiling or will the Kerdi membrane help create a flush surface for better tiling? The kerdi will follow the lines of whatever it is installed upon. If the wall is wonky, the kerdi will be wonky. if there are just a few recesses on the wall, they could be built out to the proper plane with applications of thinset or mortar feathered into those areas. Let the thinset or mortar cure, then apply the kerdi. If the recesses are deep and numerous, it might be easier to skin the entire wall with cement board, shimming the board out as needed to get the walls in plane. 3) Should I score the painted wall before applying the thinset to help it adhere better? Yes. Especially with a semi-gloss paint. I'd recommend you go further than that and attack the paint with high-grit sandpaper to really scuff it up, or to even remove it. If you end up with a variety of materials on the wall when you are done (some semi-gloss paint residue, some thinset, some drywall mud, etc) it might make sense to prime the wall in its entirety. Let the primer fully cure, then thinset and Kerdi over the primer. You recognize yours is a messy installation based on your existing substrate. Some plaster. Some semi-gloss. Some drywall mud. Some thinset. Working off a Frankenstein base like that offers no guarantees. That's why my advice, especially considering you have that gap at the tub that needs to be bridged, is to veneer the wall with cement board. Since your walls seem to be a thin plaster veneer over blueboard and not a thick plaster over lathe or a thick multi-coat plaster, you could even strip the walls to the studs. It sounds like a lot of work, but you may very well spend more time trying to make your existing walls right without them ever really being right, versus going to the studs and making them right. Get to the studs. Shim the studs out if needed so the three alcove walls are all in plane. Cement board. Kerdi. Tile. Grout. Done. Shortcuts sometimes take longer than what seems to be the more arduous path. I HIGHLY recommend you contact Schluter tech services. I can somewhat assure you that because of the hybrid nature of your walls, they'll not guarantee you anything. But they can offer you another perspective. Good luck!