Many of the prominent museums and conservationists are perplexed with how to clean an acrylic painting and have noted that mold will grow on a painting that has been exposed to humidity and temperature rise.
Mold and mildew (mildew is when it appears on fabric or paper) need a constant source of food and ideal conditions for the colony or bloom to survive and they are: Oxygen, moisture - needed to begin digesting, temperature - certain molds will only grow in warm climates and in the absence of moisture on the surface will use the relative humidity of 80% plus to live off, organic (humic) material - necessary to feed the colony.
Mold tends to grow on dead organic properties but has been known to grow in some synthetics such as paste, adhesives and paint (and in this case it will be the binders and ingredients used in the paint production that produces the food a mold spore can grow on - best to really know your paint and it's ingredients). It won't grow on inorganic matter such as glass, concrete and metal BUT
it can grow on the dirt and oil that covers these surfaces. The canvas and wooden stretchers are ideal organic material for a colony to live on.
The problem to acrylic paint is that the film remains soft and easily attracts dirt particles and mold spores which the film may flow around and embed. The glass transition stage of acrylic (Tg) is near or at room temperature - which means that acrylic film will remain soft at a normal room temp. The paint will go soft at 60C. Cleaning acrylic has been rife with problems with restorers.
has concluded that there are no safe ways to clean an acrylic and the best way to deal with mold is preventive measures and the Tate
has just released their study on acrylics and concluded that way more study is needed in this medium.
Before I start a painting, I spray the back and front of the canvas and stretchers with a mild solution of sunlight dish detergent as it has a borate substance which inhibits mold grow. I let it dry for a few days.
If you are in the habit of leaving your brushes and knives in water, you could be encouraging the growth of mold on the bristles - very easy then to transfer the spores to the paint. Rubbing alcohol is your friend in the studio for wiping down surfaces that may be a breeding ground for mold. Same goes for pouring paint back into its original container... you may have just introduced a few mold spores into your supply. Try not to use tap water as it in all likely hood contain nutrients for the spores.
For the junior scientists who want to experiment, here is one to try: A nasty experiment to try is to gather some old paint brushes and acrylic paint, (works fastest with craft quality acrylics). Get the brushes good and paint-dirty, then swish them in a mason jar of about two inches of regular tap water. Leave it. See how it changes in color, texture, and SMELL after many days. Eventually a goo will develop on the dirty brushes and the water will turn to slime. Those same chemical reactions go on inside a painting, to one degree or another, when unclean water and low resin paint (low quality) are used.
You might find these links useful:
Cleaning paintings require extra care and some knowledge of what you are doing and with what solution you are using. Never clean a painting with bleach. Even after you have cleaned a painting be aware that the mold and mildew will never go away and you must take steps to protect the environment. The only cure for mold and mildew is to clean the surface and keep the humidity down.
I've cleaned a few paintings with a mold growth on the back of the canvas. I dried the colony out by carefully adjusting the temperature and humidity so as not to warp the stretchers and then carefully with a HEPA filter suction - "vacuumed" the mold up. Then I used a solution of 3 parts hydrogen peroxide and 5 parts water and rolled a cotton tip over the stain making sure that the tips were exchanged when dirty and kept in a sealed bag (the cotton tip is in and of itself a small problem as the cotton is an organic substance and small fibres from the tip can be left on the surface allowing the spores to feed.) Make sure you wear gloves and a mask - the gloves help prevent your body oil from being deposited on the painting as there are some molds that live off those oils.