I have s plaster wall with these plastic backspashes from the 70s that are adhered to the wall in such a way that I can’t easily pry them off without pulling the plaster off or really making a mess of things ... I’m contemplating putting up subway tile, and I’m wondering if I can just use construction adhesive and stick the tiles right on the existing plastic pieces ... I would then grout of course any ideas ?
I personally wouldn't cover up the existing tiles as the new ones will look twice as thick at the edges, and you'll have to match the old tiles exactly. It shouldn't be that hard to remove the old tiles. I would try using a metal putty knife and hammer. The putty knife should get right behind the tile and not do any real damage to the drywall behind. Then just do any surface repair with drywall compound. Then I would use a mastic or thinset depending on what type of tile you choose. I wouldn't use construction adhesive just because it literally will lock those tiles in place.
Yes, construction adhesive applied with a notched trowel should work just fine, assuming that it doesn't bleed through the grout. It won't have the grab to keep tiles from sliding down, so you'll need to use spacers if they're not self-spacing.
You'll want to have good ventilation since construction adhesive is solvent-based. You could go with the latex variety as well, but it's not as tough.