As a paint film dries, I understand that it increases in mass rapidly as it takes up oxygen, reaches a zenith and then reduces in mass at a slower rate before “leveling out” at a very minimal incline, indefinitely, when graphed on a chart. Most of this change seems to take place within a few days, depending on the rate of drying.
As the paint layer gains and loses mass, does it also thicken, expand and/or shrink?
If so, it seems to me that the farther that the film progresses through these changes before another wet paint layer is added, the less movement the next layer has to deal with, especially getting past the more sudden gain and loss of mass in the first few days. Longer and more rapid drying would seem to help in the “more flexible over less flexible” paradigm. Correct?
Thanks for your thoughts,
Richard
Hello Richard,
My understanding is similar to yours. So overall I would say ‘yes’.
However I would amend that it takes longer than a few days to go beyond the main peaks of changes.
A few weeks to reach maximum weight, then a longer logarithmic decrease to a plateau ‘in general’. (Depending on the oils and pigments). So ideally at least a month to have most changes behind.
The concern of more flexible over less flexible to avoid cracking generally means to put slower drying fresher coats over faster (already) drying ones. More than volumic espansion or shrink (which would be just a few % at most), it’s the drying time and rates differences that will cause cracking of the top coats.
So yes, but it’s more about drying time than volume changes.
Cheers,
The need for flexibility in the top layer is to accommodate for the expansion or contraction of the lower paint layer, depending upon whether or not the lower layer is gaining mass or losing it again. (Recognizing that some of the loss in mass is due to evaporation of solvent in the paint layer). Correct?
As the upper layer is cutting off direct access of oxygen to the lower layer, the mass of the lower layer is not changing (drying) as quickly as before when it was alone. Correct?
If you were to graph the mass of the two separate layers out, with mass on the y axis and time on the x axis, the appearance of the latter layer would be almost like an echo of the former layer. Whatever the lower layer does, the upper layer must be “rubbery” enough to grasp the lower one over many years or else crack, split, or delaminate. Correct?
Hello,
For the first point I think it’s somewhat incorrect.
The flexibility is to accomodate the movements of the surface when it can move (essentially canvases) and accordingly the flexibility of each layer of paint below. It’s more about all moving together the same than the inherent changes of the lower layers.
(Of course if you paint milimeters thick coats that would become true as well.)
Second point is a likely hypothesis. The thicker the (overall) layer of paint, the harder it is to get thorough drying in the core. Same as putting too thick a coat straight.
Thirs point is also quite likely. Although the evolution of weight doesn’t reflect the flexibility of the painting directly. That’s unfortunately not correlated that way. The flexibility (when dried) will come first from the oils used, then the pigments, and then the painting mediums used.
Between that and the few% involved of weight change/volume shifts, I would advise to focus on drying times/rate first.
Cheers,
Lussh (should have signed before)
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