DRY AS A BONE

by whiterabbitanimation

mr_bird_mask_head

The production of Mr Bird’s masked head has required a complex series of processes and the mask’s beak was particularly challenging both structurally and cosmetically. The beak itself was constructed from Sculpey over a lightweight core to avoid unnecessary weight, which would drag the puppet forward and downward. Once sculpted, advice was to dry brush enamel paints over the surface, to give the beak a weathered metallic finish, in keeping with the full-sized version designed and built by Seth. The process is to build up the texture and colour, by applying the darkest colour first (in this case a matt black) and then gently dry brushing coats of increasing lighter shades onto the surface. Dry brushing is deceptively difficult, as it requires quite a lot of experience to see how the coats will develop to the final finish. Initially, it feels as if you are getting it wrong, as the trick is to clear the brush of all visible paint on a rag first and then very lightly brush onto the surface in a consistent direction and pressure. Eventually, the colour does start to develop, but it takes some confidence not to load the brush with more paint to get a more immediate result! Further, enamels are notoriously tricky to work with as they do not dry but rather ‘cure’. Therefore, speeding up the drying process is not technically possible as the advice below explains:

“Technically speaking, enamel doesn’t dry, it cures. It’s a fundamentally different process. Paints like nitrocellulose lacquers (like automotive lacquers) and aqueous acrylics truly do dry. The binder of these paints is dissolved into solvent (either lacquer thinner or water), and the paint hardens as the solvent evaporates. Dehydrators can be very effective in driving off solvents- they’re obviously designed to evaporate water out of food, and evaporating solvents is very similar.

Enamels harden due to oxidative crosslinking. When enamel is exposed to oxygen, it starts a chemical process which molecularly hardens the binder- creating a sort of ‘shell’ as binder molecules link with each other. Heat and convention have a limited effect in speeding this process. I may increase the crosslinking process slightly, but certainly not in the way that it dries solvent. If you thin enamels with lacquer thinner, you’re not changing the curing time. By introducing a volatile solvent, you’re creating a mix that needs to cure and dry. But after the solvent has finished evaporating, the enamel binder still must cure- and the binder will still cure at it’s own pace.

If you want enamel to cure quickly, the best way is to spray it as thin as possible. This way as much of the paint as possible is exposed to oxygen, and it can all go to work on curing. If you apply enamels thickly, only the outside surface is exposed to oxygen, which can dramatically increase the curing time of the paint deeper in the layer. Spray enamel too thick and it is possible to keep some of the paint from ever completely curing. The outermost portion exposed to air can cure perfectly well- but this cured outer portion becomes a barrier to prevent the paint deeper in the layer from being exposed to oxygen. As a result the paint will always be soft, under a thin cured film.

This is why enamels have such strange application instructions- you must spray everything on at once (within a couple of hours), or you must wait days or weeks to apply another layer. If a layer is applied and it begins (but does not complete) curing, only to be covered by another layer, the first layer will never cure.

Of course applying many thin fully cured layers is a perfectly acceptable (if very time consuming!) way of applying enamels. But if you try sanding/polishing it, you will see quickly that each of these individually cured layers is distinct. You will clearly see where you’ve sanded through one to the layer below, rather like sanding through an onion. It makes enamels difficult to apply thickly…”

Read the full forum post here.

So it took quite some time to complete the finish to the mask’s beak! I also found that time it took for enamels to cure was dependent on the type of modelling material to which they were applied. For example, those applied to the porous, slightly chalky surface of the Sculpey cured within 24 hours per coat, but those applied to the waxy, non-porous surface of Milliput (used for the scales along the mask’s nasal bridge) took much longer. I assume that this is because the enamels were able to seep into the Sculpey surface a little, resulting in a thinner coat and therefore a faster curing time. When applied to the Milliput, however, the repellent surface resulted in a thicker coat and in turn, a longer curing time.

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