Staining in Formply
The most common stains associated with formply are:
- Rust staining
- Tannin staining
- Pinking – free phenol from the phenolic surface film.
In the writer’s experience, most staining is due to rust deposits on the deck at the time of
pouring or iron in the aggregate. These stains appear red/brown and are often mistaken
for tannin staining.
Tannin staining is the result of extractives from the wood veneer leeching from the formply
and discolouring the concrete.
Wood species with high extractive content are much less common than in times past due
to weight restrictions on formply sheets. These species have been mostly replaced by
plantation pines which have a low extractive content and do not cause tannin staining. In
the past decade, tannin staining due to wood species has become very rare.
Pinking is caused by free phenol in the phenolic surface film.
The excess phenol imparts a pink or pale red discolouration to the concrete. AS 6669
requires manufacturers to test for free phenol via the alkaline resistance test. This test is
effective in identifying excess free formaldehyde and minimises the potential for formply
with excess free formaldehyde to be supplied to the market.
How to identify rust stains
Rust stains can look identical to tannin stains. However, there is a simple method to
identify stains associated with rust or oxidised iron. This involves applying “rust converter”
to the stain. If the stain fades or disappears, the stain is caused by rust or oxidised iron. If
the stain is unaffected by the application of rust converter, the supplier of the formply
should be contacted.
