What causes joint compound to bubble or blister on drywall and how do Toronto finishers correct it?

Bubbling and blistering in joint compound almost always comes down to one of three causes: air trapped under paper tape during bedding, compound applied over a contaminated or incompatible surface, or moisture-related adhesion failure. Each has a distinct correction method, and experienced Toronto finishers can usually diagnose the cause just by looking at where and how the bubbles appear.

Why Bubbles Form in the First Place

The most common source of blistering is improper tape embedding. When paper tape is laid into the bedding coat, the finisher needs to apply enough compound underneath the tape to fully wet the paper and squeeze out any air pockets — then skim a thin layer over top to lock it flat. If the bed coat is too thin, or the tape is pressed down unevenly, air pockets get sealed under the tape. As the compound dries and shrinks slightly, those air pockets become visible blisters. This is especially common when finishers are rushing — a real temptation in the GTA market where labour costs are high and project timelines are tight.

The second major cause is applying compound over a surface that isn't compatible with it. This includes painting over a repaired area and then trying to apply more compound on top without proper preparation, applying compound over glossy or oil-based paint without scuff-sanding first, or using the wrong compound type for the application. Fibreglass mesh tape is a frequent offender here — it's self-adhesive and fast to apply, but it needs to be used with setting compound (hot mud), not pre-mixed all-purpose. When finishers bed mesh tape with pre-mixed compound, the tape flexes slightly as the compound dries, and the result is a bubbly, poorly bonded joint that will eventually crack. Paper tape with all-purpose compound is the more reliable combination for most GTA residential work.

The third cause is moisture-related, and this is where Toronto's climate becomes directly relevant. In winter, when furnaces are running hard and indoor humidity drops to 15-25%, compound dries too fast on the surface while staying wet underneath. That moisture differential creates pressure that pushes the surface layer up into blisters. The opposite problem happens in summer — high humidity slows surface drying, and if a second coat goes on before the first has fully cured, trapped moisture causes adhesion failure and bubbling. Basements are particularly prone to this year-round because of ground moisture and poor air circulation.

How Toronto Finishers Correct It

The correction depends entirely on whether the compound has fully dried or is still fresh. If you catch bubbles while the compound is still wet, a finisher can carefully slit the blister with a utility knife, inject a small amount of compound underneath, and re-embed the tape with a broad knife — working from the centre outward to push out the air. This is the ideal scenario.

On dried compound, the approach is more involved. The finisher cuts out the blistered section completely, feathering back to solid material on all sides. If the tape itself has lost adhesion, it comes out entirely and the joint gets re-bedded from scratch with fresh paper tape and a proper bedding coat. Trying to skim over a bubble without removing it is a patch that will fail — the blister will telegraph through every subsequent coat and show up clearly under paint, especially under the raking light from windows that's so common in Toronto's older homes with large, low windows.

For repairs on existing walls, the surface needs to be scuff-sanded, cleaned of any dust or grease, and primed with a PVA drywall primer before new compound goes on. This is the step most DIYers skip, and it's why bubbles reappear after a repair looks good initially.

Practical Prevention

A few specifics worth knowing: compound should be applied in coats no thicker than about 3mm at a time. Thicker applications trap moisture and are far more prone to bubbling and cracking as they dry. In Toronto winters, running a humidifier in the work area to keep humidity around 40-50% dramatically reduces the risk of surface-drying issues. In summer basement work, a dehumidifier and proper cross-ventilation are equally important.

If you're seeing widespread bubbling across a freshly finished room rather than isolated spots, that's a sign of a systemic problem — wrong compound, wrong tape, or environmental conditions that weren't controlled. That warrants a conversation with the contractor before any additional coats go on.

If you're dealing with persistent blistering on a current project, Toronto Drywall Installers can match you with an experienced local finisher for a free assessment — find professionals through the Toronto Construction Network directory at torontoconstructionnetwork.com/directory?trade=insulation.