Paint Spoilage: Bacteria and moulds in decorating paint

'Gone off' and smelly paint has made the headlines recently with a well known paint manufacturer and reseller being inundated with complaints regarding spoiled paint. The paint in question had recently had a preservative removed which in turn likely caused bacterial growth and an unpleasant odour. Now customers have to redecorate and the manufacturer has reinstated the previously emitted preservative.

The thing is, this isn't a new problem. Microbiological spoilage in paints with specific emphasis on water based paints has been an unfortunate side effect of using raw materials that are prone to contamination. Water alone is a perfect breeding ground for micro-organisms and due to this, both preservative use and microbiological testing have been undertaken in the paint industry for many years. High profile cases like the above are rare but can prove quite costly to companies in terms of bad publicity, drops in sales and compensation payouts. 

As well as the ammonia smell, contaminated paints can also smell like sulphur (rotten egg odour) due to the presence of sulfate reducing bacteria. 1 However, bacteria isn't necessarily the major culprit as fungi has been assessed to be the most prevalent paint spoilage organism. 2

In addition to foul odours, there are other side effects of bacterial / fungal paint contamination such as:

Visible bacterial / fungal growth
pH changes
Allergies / infections (human health implications)

Some changes to paint preservative limits has led to manufacturers to remove certain preservatives completely as they deem that the new limits set will no longer make these preservatives effective. The preservatives included in the new regulations are methylisothiazolinone (MIT) and methylchloroisothiazolinone (CMIT). However, other preservatives may have limits set in the future. 3

Whether certain preservatives are included in the paint or not, stringent microbiology testing protocols are recommended to test for contaminants and assess the efficacy of any in can preservatives.


1 & 2. Contant, S., Caritá Júnior, G., Machado, P.F.M.P.B. and Lona, L.M.F., Evaluation of the effect of dry-film biocides on paint film preservation using neural networks. Brazilian Journal of Chemical Engineering (2010).



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