Why Do Oil Based or Alkyd Enamel Paints Go Yellow?

Why Do Oil Based or Alkyd Enamel Paints Go Yellow?

It usually becomes most noticeable when you go to touch up or paint a new area close by. You will see over time that your nicely coated oil based enamel finish has become an old antique yellow colour. It's very common and will happen particularly if the areas are in a dark room with not much natural light. We often try to match old enamel colours for our customers and are still presented with an aged colour that has gone sometimes two or three times darker. When you compare this old painted colour to the existing colour swatch it will be quite different. The most successful solution is to repaint rather than touch up. Even with our computer colour matching system, we can have variations.

This will only happen with OIL BASED ENAMELS NOT WITH WATER BASED ENAMELS.

So WHY does it happen?

Oil based enamel paint contain an alkyd resin which partly
consists of an oil from a natural source, such as vegetable oils. Oil based enamels are still used in areas
around the home like skirting boards, cabinets and doors, due to the good gloss and appearance, block
resistance and hard wearing characteristics. This toughness comes from the curing reaction of the oil
with oxygen from the atmosphere over a long period of time. One effect of the curing cycle of the oil
is yellowing, which occurs normally and increases over time. This yellowing is particularly noticeable in
clear varnishes and white or light coloured paints. The yellowing of oil based enamel paints has been
established for decades and is widely known in the painting industry.

All oil based enamel paints will yellow over time, however there are factors which can accelerate and
make this yellowing significantly worse, particularly in the first few months after the paint has been
applied. It is known that after an oil based paint has been applied, if the temperature is low and the
humidity high and if it is exposed to ammonia, the level of yellowing of the dry film will be higher than
standard. The two most significant sources of ammonia causing the yellowing of the oil based enamel
are waterbased acrylic paints and cleaning products.
The other factor known to accelerate yellowing of oil based enamels is a low level of either natural or
artificial light. This is known as dark yellowing. This can occur in areas like the inside of cupboards,
behind doors that are kept open (such as cavity doors slid permanently into the cavity) or wherever the
dry coating is permanently shaded. Unlike exposure to ammonia, dark yellowing can be reversed by
exposing the yellowed enamel to sunlight.

How to avoid or minimalise yellowing.

All oil based enamel paints will yellow over time, Haymes Paint recommends the following to minimise
excessive yellowing, particularly in white or pale coloured oil based paints:
(1) Do not apply the oil based enamel when the humidity is high and the temperature is low
as the curing of the product will be affected.
(2) Ensure good ventilation and natural light in the area where the oil based enamel is being
applied to ensure good curing of the film and avoiding the potential for dark yellowing.
(3) Common paint industry practice is to apply enamels to doors and trim areas prior the
application of the broadwall areas with water based acrylic paints. If this process is
followed, the enamel paint must be allowed to cure for at least 48 hours, with the rooms
being well ventilated, prior to the application of the water based broadwall paints. Best
practice is to apply the oil based enamel after the water based acrylic paint. Allow the
water based paint to dry for at least 48 hours and up to two weeks in a well ventilated
area prior to applying the oil based enamel.
(4) Avoid using cleaning products with high levels of ammonia around oil based enamel
paints that have been recently applied. You can determine if the cleaning product
contains ammonia by reading the label section of the cleaning product that lists the
ingredients.
(5) Avoid applying oil based enamels to surfaces where there is going to be little to no light
and do not keep doors or other areas painted in oil based enamel permanently open or
shaded, e.g. cavity doors permanently slid into the cavities or internal doors left
permanently open.

How to rectify or reverse the yellowing.

If the yellowing of the oil based enamel was due to the absence of natural light, then the yellowing
can be reversed over time by exposing the paint work to light. This reversal is not instantaneous and
depending on the light level in the room could take some time.
If the yellowing of the oil based enamel is due to poor ventilation and exposure to ammonia, then the
enamel should be allowed to cure sufficiently, then lightly sanded and repainted with a non-yellow
water based acrylic enamel product like Haymes Ultratrim Acrylic Enamel High Gloss or Semi Gloss.

What's the alternative?

If an area such as a door or skirting is going to be painted with a white or pale colour and colour and
yellowing is an issue, then water based acrylic enamel technology like Haymes Ultratrim Acrylic Enamel
High Gloss and Semi Gloss can be used as an alternative to the oil based products. The alternative to
the oil based clear is Haymes Aqualac GP Clear. These are acrylic paints will not yellow like the oil based
enamel products.
Haymes Paint has an ongoing development process in the area of trim products to ensure products
with excellent film toughness and low yellowing.

Additional information: (or download pdf doc below):

Additional information on the yellowing of solvent based alkyd enamel paints is available from the
following organisations:
http://www.apmf.asn.au/Yellowing%20of%20Light%20Coloured%20Enamel%20Paint.pdf
http://www.mpav.com.au/public/019-Yellowing-Enamels.pdf

Disclaimer.

The information provided is correct at the time of preparation; however it is the responsibility of those
using this information to check that it is current prior to specifying, recommending or using products
contained in this information. Because use conditions and applicable laws may differ from one location
to another and may change with time, those using this information are responsible for determining
whether products and the information in this document are appropriate for their use and for ensuring
that workplace and disposal practices are in compliance with applicable laws and other government
enactments. Haymes Paint assumes no obligation or liability for the information in this document.
No express warranties are given except for any applicable written warranties specifically provided by
Haymes Paint. All implied warranties including those of merchantability and fitness for a particular
purpose are expressly excluded.

 

 

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Tags: Oil Based Paints, Yellowing Paint

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