How do you remove liquid nails from a fiberglass bath tub and surround?

Unfortunately this will be difficult at best. Both are plastics. Spray foam is very tenacious. It bonds really well to most things (including tubs). That said; I have been successful at removing overspray from many surfaces over the last 20+ years.

Now, depending on the level of overspray, you might be satisfied with the results achieved using a 3-M Clay Block and their clear liquid spray polish.

Use a soft rag and liquid polish to gently remove the foam splatters. Spritz' the area with polish and "Gently" start rubbing the area with the clay block. This will produce decent results.

IF the damage is worse, or you are still plagued with "Spots", then your next approach (much more aggressive) may be needed.

Once you have clay blocked the area, use 1200-1600 grit "Wet" sand paper with cloth backing. VERY GENTLY, with the liquid polish; wet the surface and sand the affected area. Follow this process with a polishing compound. Like that from DuPont.

Remember; doing any of the above could make the tub look much better. If you still aren't satisfied; There are people that can Re- gel coat the finish. That process may cost as much as a tub, but may be cheaper than the cost to R/R it.

Good luck.

Dave