I have this habit of setting aside half-finished paintings when I get that feeling that something about it is off. "Aha!" moments sometimes come when you've stopped obsessing over something. Sometimes it would take me days or weeks before I would get back to a particular painting. This one somehow worked its way to the bottom of my pile and I forgot all about it. Almost a year has passed since I last touched it. When I tried working on it again to finish it, spots started appearing wherever I wet the paper.
Reminded me of salt effects. This one however, is caused by sizing gone bad. We have touched a bit on sizing in a previous post. (Watercolor Papers and Sizing). But basically, the idea behind sizing or the addition of gelatin to watercolor paper during its production is to make the paper more workable with watercolor. Sizing allows you better control over your watercolor as it decreases the tendency of the paper to absorb liquids and paints like tissue paper. Between the paper pulp and the gelatin, the gelatin would be the first to go stale with old stock paper. Signs of this would be the appearance of spots that don't go away after a wash or after an application dries. Another would be when you discover areas that resist any application of paint you put down on it. 
There are several ways to hasten the deterioration of the sizing. As demonstrated in this ruined painting, one way is by wetting a painted watercolor that has lain undisturbed for months. You can also hasten the deterioration of new paper using the same principle. The moment you wet a watercolor paper, its sizing gets disturbed. Maybe water acts as a catalyst. This is the reason why you are advised not to stretch paper in big batches. Stretch only what you think you will be able to use within 2 or 3 months. That's just my estimate. In our weather where heat can be more than the usual, it is always more prudent to stretch only what you think you will use. The window of usability may be different in your environment and you should let experience guide you. 
A sign that this was not bad paper to begin with is how the undisturbed, previously painted part is free of spots. Check the picture. You wouldn't know that the paper's sizing has gone bad if you do not re-wet the paper. When you find yourself itching to touch up a painting years after it is finished, remember what happened here. Not all old stock paper go bad. I use Arches cold pressed watercolor paper in rolls. One of the most economical way of buying paper is by buying it in rolls. Because you cut to size, you minimize wastage of excess paper. As long as you do not get the roll wet and observe proper storage, the paper stays usable for years. Proper storage includes storing it in its original wrapping and in the box it came in and with the crumpled paper fillers still inside. Do not put this container/box in an area prone to dampness such as near windows, bathrooms, basements. Also keep it away from direct heat or sun exposure so the paper inside does not get heat-baked.
You do need to recognize what stale sizing on paper looks like, specially when you like to take advantage of art supplies on sale. Some stores put their older stocks on sale just to move the items and to make way for newer supplies. Some of these discounted paper will still be good for use but many might have sustained handling or storing damage. For this reason I prefer to spend on paper and get new stocks. You're not really saving on money if you get bad paper. But sometimes good bargains are hard to resist so if you must, at least learn to discern the appearance of stale sizing to help you shop wiser.
I wonder if you can see the very pale yellow spots. That is how spoiled sizing looks on paper that has never gotten wet. Just turned bad over time. (I keep samples of everything.) But don't go hunting for spots where there are none. Even new paper looks a bit uneven because of how the lighting plays off against the hills and valleys of the paper. The surest way to check is to wet the paper.
This is the same paper, now wet. Some new paper may have this tendency but on a very mild scale and if the paper dries without any marks, your paper is still good. For tips on how to minimize this tendency, check out my previous article Watercolor Papers and Sizing
There are times when you would come upon a defective batch of paper. The best way is to contact the seller and if no action there, the manufacturer to see if you can get a replacement. Might be wise for you to do a little sleuthing online to see if other buyers have been complaining about certain batches. You will have a stronger claim if your paper is from the same batch. But do not be too quick to blame the suppliers. Sometimes we may be unaware that we are doing something that harms the paper.
You do need to recognize what stale sizing on paper looks like, specially when you like to take advantage of art supplies on sale. Some stores put their older stocks on sale just to move the items and to make way for newer supplies. Some of these discounted paper will still be good for use but many might have sustained handling or storing damage. For this reason I prefer to spend on paper and get new stocks. You're not really saving on money if you get bad paper. But sometimes good bargains are hard to resist so if you must, at least learn to discern the appearance of stale sizing to help you shop wiser.
I wonder if you can see the very pale yellow spots. That is how spoiled sizing looks on paper that has never gotten wet. Just turned bad over time. (I keep samples of everything.) But don't go hunting for spots where there are none. Even new paper looks a bit uneven because of how the lighting plays off against the hills and valleys of the paper. The surest way to check is to wet the paper. 
This is the same paper, now wet. Some new paper may have this tendency but on a very mild scale and if the paper dries without any marks, your paper is still good. For tips on how to minimize this tendency, check out my previous article Watercolor Papers and SizingThere are times when you would come upon a defective batch of paper. The best way is to contact the seller and if no action there, the manufacturer to see if you can get a replacement. Might be wise for you to do a little sleuthing online to see if other buyers have been complaining about certain batches. You will have a stronger claim if your paper is from the same batch. But do not be too quick to blame the suppliers. Sometimes we may be unaware that we are doing something that harms the paper.
You can artificially ruin the sizing of the paper by soaking it too long under water. Prolonged immersion may be the culprit why some sizing coagulate in spots. This used to happen to me when I was a beginner. Thinking more is always better, I would leave watercolor paper soaking for as long as 20 or 30 minutes before stretching it. The suggested submerging time is only a few minutes. Only long enough for the paper fibers to get wet. If you soak it too long, even before you staple it down, you can tell you've ruined the sizing by the appearance of slightly darker spots on your paper like in the sample above.
Also, not all paper that develop slight spotting when wet are damaged paper. Even new and undamaged paper may develop these slightly darker spot discoloration when wet. But these would tend to disappear as the paper dries. If you can't tell any spotting on the dried, stretched paper, your paper is good to paint on.
That's it. Thank you for reading.
Also, not all paper that develop slight spotting when wet are damaged paper. Even new and undamaged paper may develop these slightly darker spot discoloration when wet. But these would tend to disappear as the paper dries. If you can't tell any spotting on the dried, stretched paper, your paper is good to paint on.
That's it. Thank you for reading.
