▽ +09.00Problem · Roofs
Bubbles and pinholes in PU waterproofing
Small bubbles or pinholes appear in the polyurethane layer while it cures or shortly after.
What you see
- Small bubbles across the surface
- Pinholes that go through the coat
- Bubbles concentrated over cracks, joints or porous patches
- A tacky or foamy texture
Possible causes
Moisture in the substrate
Polyurethane reacts with water and releases carbon dioxide.
Outgassing from porous concrete
A warming surface drives air out through the fresh coat.
High humidity during cure
Especially in the evening and at night.
Thick coats or poor mixing
Air trapped by fast mixing or heavy application.
No primer, or the wrong one
The primer seals the pores and tolerates moisture where it is designed to.
How it is diagnosed
The diagnosis confirms the cause before any repair is chosen.
When did they appear?
During cure (moisture or air) or later (trapped water).
Moisture readings
On the substrate around the affected areas.
Application records
Time of day, temperature, humidity, primer and coat thickness.
Cut a sample
Check the thickness and whether the bubbles pass through.
Possible solutions
The fix follows the diagnosis; these are the common options and their systems.
Sanding and re-coating
For surface bubbles: abrade, prime as the system specifies and apply a sealing coat.
Systems
Removal and re-application
Where pinholes pass through or the bond is poor, once the substrate is dry.
Systems
A moisture-tolerant primer or another system
Where the substrate cannot be dried.
Systems
What not to do
- Applying while the slab is heating up in the morning sun
- Coating a damp slab
- One thick coat
- Covering pinholes with another coat without preparation
General educational content; final values and requirements follow the manufacturer’s TDS and the approved project specification.
Common questions
Why does PU bubble on concrete?
Moisture in the slab reacts with the PU, and air escapes from porous concrete as it warms.
Technical inspection
Need a diagnosis for your case?
Send photos, a description and the location; our engineers identify the cause and the fix after an inspection.