Painting big!

It's going well, Linda. A remarkable painting. And I'm also learning from this thread - alkyd whites and being able to go quite far with acrylic before adding oil. If it's at all possible, to see a photo of the completed piece in situ, would be great.

I was surprised at how far Linda took the first stages using acrylic… I’m no expert here on such matters, unlike Robert, but I don’t think it is particularly advisable, but I suppose it depends on how thick the acrylic has been applied! Time will tell of course, hopefully the two layers won’t part company with each other in the future! I’d welcome Robert’s opinion on this…

Well - there's not a lot of room for doubt now that while oil painting on acrylic primer - often referred to as acrylic "gesso", though it isn't - is safe, and better than the old rabbit-skin glue and whiting grounds (which do work well, but can have longer term issues), there's much less confidence to be had in oil painting over regular acrylic paint.  It's long been conventional wisdom that you can paint in oil over acrylic, but not the other way round: that's true, but only in the sense that acrylic over oil will just peel off, whereas oil over acrylic won't just fall immediately off the canvas. That, however, isn't the same thing as saying that the long term adhesion between layers can be guaranteed: especially on a flexible surface.  Cracking in time is likely, because of the weakness of the bond; delamination, depending on the pigment composition and enormous differences in drying rates, is a risk.  How great a risk depends on so many imponderables, as many as apply to the injudicious use of Zinc Oxide in oil (and only in oil): it can crack and delaminate, but - doesn't always do so.   The fact that these things can happen doesn't mean they will; but the possibility is enough to dissuade me from using Zinc White (Oxide) oil paint, or from using oil over acrylic.  Alkyd oil is much more likely to be a safe choice.   Will it matter in this case - probably not, because a) the substrate isn't flexible: you don't get much more rigid than a door, though even that depends on what it's made of: you'd certainly want to keep whatever forms the substrate  dry; b) the overall oil paint is applied as glazes; if a substance like Liquin is used in making those glazes, that's an alkyd medium, and it's more likely to form a glue, if not necessarily a bond, to hold layers together.  Heavy layers of added oil would not be advisable.  Research on all of these things is ongoing - while we have hundreds of years of experience with oil paint, we have only around 50 years' worth with acrylics; certainty is not guaranteed.  The MITRA pages on the University of Delaware website provide a great deal of current information, and of the paint manufacturers, the most research so far as I'm aware is being carried out by Golden acrylic paints, who also make Williamsburg oils, and Rublev, with their Natural Pigments website.  George O'Hanlon and Tatiana Zaitseyeva of Rublev/Natural Pigments, are trained and highly competent paint-makers, analysers, and chemists; the Golden Company provides excellent information and researches its products with great care - as is proved by the fact that over the years they have withdrawn some of their products or issued advisory notices on their use.  Not every art supply company can claim this level of investigation or the competence to conduct it.   In short, I don't think Marjorie need worry overmuch about her project, which I take to be a bit of fun with more serious overtones, but at the same time I'd have suggested alkyd as a better bet with it, and no, I would not paint in oil over acrylics for fear of long-term damage (which isn't to say that my stuff is worth preserving!  But if it were, and as yours is, Alan, I'd avoid overpainting in oil).   

Robert, it’s not my project - it’s Linda’s. A slip of the finger when typing.

Age, that's what that is - Marjorie, Linda, Paul, Alan..... it's all too much: I need a lie-down and a nice cup of tea. 

I do quite often draw in the initial frame work and block in my darks using a thin watery acrylic, but nothing more than that! Thanks for the ‘above average’ comprehensive explanation Robert… I expected nothing less from you!

Apart from using an acrylic gesso, I've never used acrylic for any sort of block in. If I'm working in oils, it's oils start to finish. I do appreciate that blocking in with acrylics then applying oils is a much quicker process.

Thanks for this extra information, Linda.  I agree, this afternoon is best hunkering down from Storm Bert and turning up thermostat to watch Powell & Pressburger movies on the telly!

Linda - oil does dry, but takes a looooong time to do it, depending on its thickness and also on individual pigments, some of which are far oilier than others; it's this, primarily, that causes issues about stability over other surfaces: acrylic sets, and doesn't move (unless you freeze it, which you won't be doing).  Oil, on the other hand, keeps responding to atmospheric conditions - it contracts and expands; obviously this isn't a massive movement, but then to cause hairline cracks, it doesn't need to be.  You'll know well that many old oil paintings do have such cracks, because there never was a perfect surface - or if, say, copper is perfect in terms of cracking, well; who could afford it?  (And anyway, it probably isn't.)  So we used cloth, and cloth itself moves (I'll swear my waistbands shrink.....).  In the normal course of events, we don't need to worry about this; if everything moves a bit, the fact that oil remains flexible for a long time can be a positive advantage.  But acrylic doesn't - or to the extent it does, doesn't do so at the same rate as the oil laid on top of it.  So actually it does flex a bit - as you suggest; but it's the rate of flex which can cause cracks.  (The fact that you can roll up canvases painted with acrylic proves it's not anything like as rigid as some think - it's the differential that's the potential problem.) I really wouldn't worry about this on a rigid surface; I'd worry about it a lot more on stretched canvas, but I bet your painted door will be around long after I'm pushing up the daisies; as a general rule (and as you know, my answer was to Alan's point rather than specific to your painting) I wouldn't want to glaze in oil over acrylic: but there are major risks, minor risks, and calculated risks - and I'd put yours as hardly in even the middle category and am sure you thought about what you were doing, so that's the "calculated" bit taken care of.  I can't see any good reason why it wouldn't work, but if you take on a project like this again (I imagine once would be enough!) I would suggest you try alkyds.  Alkyds are so near to oil in appearance that you might not want to glaze over them at all, but if you did it should be safe as houses, and maybe save you a job - I always welcome things that can do that.... And I got your name right this time!  You see, progress IS possible.