View Full Version : Waiting Until a Previous Layer is Dry Before Adding Another Layer?
12-15-2009, 07:34 PM
Does anyone know the details here? It seems that someone recommended either adding another coat more or less immediately (which seems like a sound practice, but if anyone knows of potential problems, please feel free to post) or waiting for the previous layer to dry.
What are the concerns here? Why couldn't you just add another layer at *any* stage -- anywhere from very freshly applied paint to fully cured paint?
Can surfactants or other substances cause adhesion problems if they are present on the surface?
Dry, empty pores vs pores that still contain unevaporated fluids might be another factor.
Another aspect seems to be the crosslinking that might be able to take place if another coat were added while the previous coat was still drying or curing. There might be additional interlocking of the layers if the previous layer were not completely cured....
Any thoughts or insights on when to add another layer, and what problems might arise if it is done at various different stages in the drying process?
12-16-2009, 06:53 AM
I mostly paint wet in wet, but if a layer is already dry that's no problem. IMO you can add layers in every drying stage.:)
12-16-2009, 09:59 AM
I paint wet into wet all the time as well and the only down fall can be that you go into the wet area too many times and you can end up with MUD or a very grayed color. Wet into wet is a lot of fun - go in quickly - get out quickly and don't go in too many times.
12-16-2009, 10:20 AM
My personal preference is to paint wet on dry only because I use thin translucent glazing layers and repeat over and over again to achieve the effect I want. (Think of layers of glass being laid one on top of the other). I even use a hairdryer to speed the process up because I'm not a patient woman. lol.
12-16-2009, 02:36 PM
[Can see needs get a hairdryer... :eek::lol::lol::rolleyes:]
am the same - wet on dry... and it not usually take long to dry, especially doing it the glazing way [impasta style, prob some time, what with the thickness]...
12-16-2009, 07:00 PM
Sometimes if im doing some glazing, and the previous thin translucent layer isn't dry, the following layer will just lift the previous one off.
12-16-2009, 11:49 PM
You need to be pragmatic. What are you painting and what is the next step you are trying to do.
If you have just blended gentle gradations or applied a translucent glaze
THINK TWICE ABOUT TOUCHING AND DISRUPTING A CAREFULLY APPLIED SURFACE
USE A HEAVILY LOADED SOFT FAN BRUSH AND FLOAT ONTOP OF WET PAINT
OR LET IT BE (beatles) UNTIL IT DRIES or atleast sets with some skin
you may cause:
rip the surface maring some pristine effort
You may do it perfectly and have alot of fun.
12-17-2009, 03:25 PM
Wet-on-wet okay. Wet-on-dry okay. Wet-on-tacky and not quite dry. NOT GOOD! You'll probably pick the underpaint up, maybe right down to the canvas. Particularly true with one of the funny "open" recipes. Been there, done that!
crosslinking I don't follow this. I didn't know that acrylics crosslinked. Pls enlighten me on this! References, pls! Thanx.
[BTW, why are you asking this Q? You're one of the tech experts on this forum! Is this a test?]
12-17-2009, 04:21 PM
I add layers whenever I want to except for glazing, then wet on dry.
I did find a problem with the Interactives remaining open for longer - I discovered I'd got used to the quick-drying variety!!
I read a thread somewhere here yesterday that said all layers bind into one - they don't remain separate ... sorry I can't remember which thread!:eek:
12-17-2009, 06:02 PM
Thanks for all the responses. Anything else anyone wants to add would also be of interest.
Here is an excerpt and a ref:
References to cross-linking in waterborne acrylic emulsions are intermittent. Cross-linking can occur at
three stages: during the polymerisation/production of the raw polymer resin; during drying/coalescence of
the paint film; and during aging (both natural and accelerated) of the dried film. Acrylic emulsions can be
formulated to undergo varying degrees of cross-linking during drying, depending on the end use of the
product, using additives called ‘cross-linkers’; however, these are not thought to be present in artists’
emulsion paints [19,90,55]. Instead, the high MW of the polymer is enough to provide high film strength
from chain entanglement [53,55,91]. It has been reported that during aging, a film can cross-link and
oxidise as a result of photo-degradation  and from the effect of residual surfactant , however, while
Chiantore (76) detected cross-linking in sample films, both before and after aging, oxidation products were
not found. The principal consequences of cross-linking are an increase in brittleness [93,65] and hardness,
which may actually improve the film’s resistance to dirt pick-up and abrasion .
Chain entanglement is a related issue.
12-17-2009, 06:42 PM
Thanx for the references, Nilesh. Years ago, we ran DTMAs on all types of dried acrylic paints, from ordinary house paint to the highest quality artist acrylics. We found no indication of crosslinking, even on 20-yo Florida-weathered house paint. If there was any, the effect was below the accuracy of the measuring device. Usually if there is crosslinking, it would occur while the molecules were mobile (e.g., sufficiently above Tg). From our measurements, even highly plasticized or modified acrylics had Tgs near or above room temperature. I wonder if the reported "oxidative crosslinking" wasn't just oxidative degradation, resulting in rigidifying of the backbone and embrittlement of the films or maybe just reaction with residual emulsifiers. I'll certainly check out the references, though. As I'm sure you are aware, chain entanglement is not crosslinking. I'm not convinced that chain entanglement is as important as High MW. The backbones of acrylics, even with high levels of ethyl acrylate, usually do not have LCBs or large pendant moieties. Anyway, an interesting discussion!
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