SPOTS ON PAINT.
Poor lumber and thin painting are often the cause of spots on paint, especially on two-coat work. On cross-grained and other extra-porous places more of the oil sinks into the wood than on the general surface, and the result is flat places in the paint, which fade sooner than the glossy paint; hence, the work looks spotted.
To provide against this kind of spotting use more care in priming and see that all extra-porous places are well filled with the prime coat, or touch them up before the second coat goes on. A little extra work with the brush when putting on the prime will save trouble.
Another cause may be traced to the practice of putting on a coarse dark priming coat, which will show through in places where the paint is thinnest.
Mildew, or fungus growth, is another cause. This sometimes comes from the use of too much japan, poor or fat oil, or when the paint dries tacky or soft.
Adulteration of linseed oil with mineral and other non-drying oils, has a tendency to make paint dry soft. Linseed oil, kept for a few days in an old sour tank or in an old rancid can in the paint shop, is liable to cause fermentation to take place, which may result in mildew in damp weather in shaded places.
When an oil can smells sour, or there is a deposit of foots at the bottom, it is unfit to keep oil in.
Another cause of spotting may be found in insufficient and improper brushing or spreading the paint; especially the priming, which requires as much care in putting on as any other coat on the job.
For instance, here is a job which shows “laps.” Now, if this prime is right when it is put on single, it is wrong when it is put on double, because, where the laps are, the work has at least one more coat than the balance of the job, hence the paint is liable to fade spotted.