painting

Navagator

WE used a Zinser bulls eye 123 primer over some old stained and varnished woodwork. The primer says you may recoat the primer in one hour, but it takes 7 days to reach full hardness. Should we wait the 7 days to apply the finish coat to help the paint bond better, or can we go by the normal 8 hours or overnight, then apply the finish coat?

Allison1888

Hopefully you scuffed up the wood a little to help the primer adhere. I think you'd be fine to wait a day or two, not seven. (Assuming there is not excess moisture around)

Navagator

We used a product called paint bond which is used to scub the varnish and remove some of the gloss, even though the Zinser primer said it was not necessary. I checked the primer, four hours since I finished the primer coat and it seems to be dry.

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.

Faron

Mickster40

Is paint bond something you can use versus sanding? If so what brand and do painters recommend it?

Nestor_Kelebay

I'm not going to tell you when to topcoat because I think you've gotten good advice from the professional painters in here who've told you that you can top coat within a few hours of applying the primer. I'm going to explain why it takes 7 days for the Bullseye 123 primer to achieve it's maximum hardness.

I don't think Zinsser is saying to wait 7 days before recoating. They're just saying that it takes 7 days for the plastic the paint film is made of to reharden completely, and that to me seems like a bit long, but not unreasonably long.

You see, latex products like Bullseye 123 consist of a slurry of HARD CLEAR plastic particles (called "resins") and solid coloured, clear or white particles (called "pigments") suspended in a mixture of water and a low volatility water soluble solvent called a "coalescing agent" or "coalesing solvent". When you spread the paint on the wall, the water evaporates first, and those hard clear plastic resins find themselves surrounded by that coalescing solvent at a steadily increasing concentration. That coalescing solvent dissolves (kinda) those hard clear plastic resins so that they get soft and mushy. They get so soft that the weak forces of capillary pressure and surface tension are enough to cause them to pull on one another to form a continuous film of soft mushy plastic on your wall. (It's the same physics that causes tiny droplets of water in a clowd to coalesce into bigger and bigger droplets until they're so heavy that they fall as rain.) Anyway, this film forming process only takes 5 or 10 minutes or so, but over the next 2 or 3 days the coalescing solvent evaporates from the latex paint or primer film, and those hard plastic resins harden back up again to their original hardness as the coalescing solvent evaporates out of them.

I've always presumed that it only took 2 or 3 days for the coalescing solvents to evaporate out of the paint film because typically by 3 days after painting, that "newly painted smell" is beginning to wane. Maybe that's only what we humans percieve because our sense of smell isn't very good. Maybe a dog would continue smelling that newly painted smell (which is the smell of the coalescing solvents) for a full week after painting (?).

Anyhow, that's almost certainly why they're saying that it takes a full week for the 123 to achieve full hardness, but if you're going to be top coating with another latex, then the coalescing solvents can evaporate through a top coat of latex paint just as easily as they can evaporate out through the Bullseye 123. And, I expect they can readily evaporate through a fresh coat of oil based paint too because it's common for people to paint over latex primer with oil based paint only a few hours after applying the latex primer.

Bonding isn't related to hardness in a latex paint or primer. The bonding occurs during the film forming process when the latex resins are all soft and mushy and saturated with coalescing solvent. So, your paint is bonded as well as it's going to bond within a half hour of painting. However, you have to wait for the coalescing solvents to fully evaporate from the paint film before the plastic film on your wall is as hard as the plastic resins were before they formed a film.

Film formation in oil based paints is a completely different kettle of fish, and nothing like the film formation process in latex paints at all.

Nestor_Kelebay

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.)