Spalling on Edges
Rough filling in the corners
Indicates that there was a poor bond between the first coats and the backup shell in these corners; causing primary coats to pop off (areas of primary coats popping off of edges)
Causes of this defect are evident in the shell portion of the process; to cure, R&R recommends taking the following actions:
Slurries need to be applied in an even, uniform coat. When the slurry viscosity is too high or the tree is not drained properly, heavy layers of slurry are left behind. These heavy areas do not dry properly and leave a soft, poorly cured moisture pocket behind. When the part is dewaxed, this moisture tries to escape and causes the primary coats to pop off. This typically occurs in the second prime.
Poor slurry control.
High binder solids in the primary slurry will inhibit drying, causing the slurry to dry on the surface and entrap moisture underneath. This moisture will try to migrate out too fast during dewax, causing spalling.
Prewetting is used to stop the previous shell layers from absorbing excess binder from the slurry. The shell will shine after the prewet drains off. Excess prewet can also be washed into an area by the next slurry layer causing a liquid pocket. This pocket becomes a void when the binder evaporates leaving a weak area.
Note: It is also possible that fines in the stucco or excessive stucco coating is causing adhesion issues between the primary and secondary slurry layers; however, spalling in corners is typically related to residual moisture entrapped in the shell layer and bursting during dewax. Stucco-related adhesion issues are described on scabbing/delamination defect.