Originally Posted by Faron79
That's a good time range to wait b4 the 1st paintcoat.
"Full hardness" means that the complete "bonding"/curing-out of the primer-layer can take a week or more.
The paint layers themselves aren't considered cured/hardened-off for ONE MONTH.
I'm not sure what you mean by "bonding/curing-out", but for latex paints the bonding happens during the film formation process which only takes a few minutes. Basically it happens during the time the last of the water is evaporating out of the latex primer or paint and the resins are at their softest and mushiest.
Normally, the film formation process for latex paints is the way I described it in the last post. There are, however, something called crosslinking acrylics. They form a film just like normal latex paint resins, too, but once that film is formed, crosslinks form between all the neighboring latex resins, and that cross linking will take up to a month or so. Typically, you find crosslinking latex resins in top quality latex paints and where higher hardness is needed, such as latex floor paints.
So, when you say that it takes a month for a latex paint film to "fully cure", that only applies to those latex paints made with resins that cross link with their neighbors after the normal film formation process is complete.
In OIL BASED primers and paints, which form a film entirely by crosslinking (with oxygen from the air), that crosslinking continues indefinately, albeit at an ever decreasing rate. Theoretically, a drying oil or alkyd paint that was first put on the wood siding of a house will still be getting harder 100 years later when that house is bulldozed and the siding is in the landfill site. But, I wouldn't argue with anyone who said that an oil based paint is very close to it's ultimate hardness within a month.
So, when you say that it takes a full month for latex paint to fully "cure", you need to explain that the film formation process only takes a few minutes, and the subsequent hardening as the coalescing solvents evaporate may take up to a week, but if the paint is made with a crosslinking resin, the subsequent crosslinking will take a full month or more.
Most latex paints do not crosslink. Most just go through the normal film formation process and stop there. In top quality latex paints, you need higher strength and hardness of the paint film because they will use harder extender pigments. Extender pigments are the stuff they add to lower the gloss of the dried paint film, to make it dry to a flat or eggshell gloss rather than a high gloss. In less expensive paints, these will typically be made out of calcium carbonate (chaulk) or magnesium silicate (talcum powder) ground to different fine-ness. However, top quality paints will use harder extender pigments (like ground up silica sand) because these improve the scrubbability of the paint so that it doesn't lose it's gloss when you scrub hard on it to remove a stubborn mark. It's very much analagous to sand paper; to make good sandpaper that doesn't wear down easily, you need BOTH hard sand and a strong glue to hold that sand firmly in place. If you have either soft sand or soft glue, it ain't gonna work; BOTH need to be strong and hard for the paint to stand up to hard scrubbing. So, in top quality latex paints they will use ground up silica sand as the extender pigment and a crosslinking acrylic resin as the binder so you get paint that will stand up to hard scrubbing without losing it's gloss.
Similarily, you need a crosslinking acrylic resin in a latex floor paint so that dirt doesn't get embedded in the paint underfoot and make the traffic lanes dirty. (Still, my experience is that latex floor paints simply aren't hard enough to stand up well on any working surface, whether it be a floor or a shelf or even a window sill. You need an oil based paint on such surfaces to provide good service.)