I was hired to make a four tier dummy cake that was decorated with royal icing. It is rock solid, but I am a little concerned about bugs. This cake needs to last for a VERY long time. Any suggestions on how to seal it?
I really don't want to disappoint the client.
Thank you all in advance for your help

You could try spraying the cake with a thin coating of aerosol shellac...I've never coated any of my RI dummies and they've lasted for well over a year w/o any bug problems.

why dont you use plaster spackle or perma ice

It shouldn't get bugs. I made the below cake 2 years ago and it's totally fine. I didn't do anything to protect it. Just leave it alone.

Agree with fairytale. You don't have to seal it. I kept a dummy cake for 10 months and it was fine .... it would have lasted even longer, but I just decided to change it out.