Why did my epoxy turn wavy or crack over a sticker or photo?

Mod Podge is not the cause. It doesn't interact with the epoxy, and we have used it under our epoxy for over 10 years without issues.

Waves usually come from using the heat gun or torch after the 20-minute working window. You have 20 minutes after pouring the flood coat to pop bubbles; after that the epoxy enters its gel stage and disturbances can't re-level, leaving waves. A fan blowing across the surface, a room below the 75°F minimum during application or the 72-hour cure, or spreading the epoxy instead of letting it self-level can do the same. If the waves follow the exact outline of one sticker while everything around it is flat, that sticker's material may simply be incompatible.

If photos bubble or crack months later on a hot day, the Mod Podge is not the source; some other component is. Wood expands and contracts with temperature, which is why we recommend sealing the underside with oil-based polyurethane. If the bottom was sealed, something else is the cause, which we can't pinpoint remotely.

The fix is the same either way: sand the surface, finishing with 220 to 320 grit, wipe off all dust with denatured alcohol or acetone, and re-pour the flood coat with every application requirement met. The new coat fills the scratches and cures clear. If you sand down to bare wood, re-do the seal coat first or the flood coat will bubble. Sanding alone leaves a cloudy, matte finish, so don't stop there.