The bubbles or blisters that appeared after applying drywall mud over a previously painted or sprayed ceiling in a foyer are likely due to poor surface preparation and lack of proper adhesion. The existing painted surface may have been glossy, dirty, or uneven, preventing the mud from adhering properly.

To rectify this issue, the affected area should be carefully inspected to determine the underlying cause of the bubbling. If the bubbles are small and limited in number, they can be easily scraped off and the affected area can be sanded down to create a smooth surface for reapplication of mud. However, if the problem is more pervasive, it may be necessary to remove all the mud, address the underlying adhesion issues, and start the process anew.

Before reapplying mud, ensure the surface is clean, dry, and free of any contaminants. It may be beneficial to lightly sand the entire area to promote better adhesion. Additionally, consider priming the surface with a high-quality primer suitable for both drywall and painted surfaces before applying a new layer of mud.

In the future, proper surface preparation is crucial when working with drywall mud over existing painted surfaces. This includes ensuring the surface is clean, dry, and free of any gloss or contaminants that may impede adhesion. By following these steps, you can achieve a smooth and professional finish on your foyer ceiling.