Hi ruscus...I just finished a mosaic on a plastic globe. This is the first one that I've done on plastic...so no history to report. But in one of my mosaic books it says that you can use plastic as a base if it is thick enough that it will not flex. It says that "if the surface flexes, all but the smallest tesserae may pop off".

I'll be anxious to see if anyone else has had experience with this.

aah...San Diego - beautiful! Lived there for about 7 years (Imperial Beach, then Chula Vista) - loved it! Am back in my home state of Florida now.

As long as it doesn't flex it should be fine. If it flexes your grout will crack even if your tesserae doesn't fall off. Roughing it up with sand paper won't hurt. If it was me I'd use GE II silicone. That should stick pretty well to the plastic & I've had good luck with it on rounded & vertical surfaces. (Some other adhesives I've tried, the pieces tend to slide down hill, but not the GE II)