Good question Shaun,
With hindsight, maybe I should have investigated such paints.. but to answer your question..

I was worried that a commercial paint may be oil based, therefore permanent, and may contain chemicals that could effect the dial, or react with the white print of the dial.
If oil based, it may need to be thinned, using white spirit or specialised thinners, something that I thought may react badly with the dial. I considered not only any instant reaction, but one that could cause a slow (2-3 week or 6-12mth) reaction.. and as the dial is 44-5 yrs old, I was very concerned about this.

One thing I have learned from my fine art days, is that when working on very small scales, the mobility of any paint is key.. oil based paint mixed with thinners is can be very mobile, and can run uncontrollably - due to low surface tension. Also, as the areas I was painting are actually higher than the dial (by a few tenths of mm), run-off was a major concern.

If the paints are modern acrylic, then this solve the thinners issue, being water based (but usually dry to a waterproof finish). But again, these paints are very permanent, and usually have a gloss finish. This can be solved with a larger working area by roughing up the surface, but I didn't have that luxury, and wanted to avoid any abrasion techniques. The other issue was that as I was working on such a small scale, dry time would be significant advanced, and as they dry, the chance of correcting or altering becomes impossible.

The other problem when working on this small scale is how any paint holds on a brush. As an example - if you take a very thin wire, like a human hairs width, dip it in paint, you will get a small amount on the tip.. now try to deposit this paint onto another shiny surface.. the surface tension holds it on the wire... the paint doesn't want to leave the wire, as it has more surface area and more surface tension.. so you need the correct viscosity.

The technique I used was similar to that of a watercolour artist.. ie. wetting the area you want to colour, and then introducing the tip of the brush (loaded with darker material).. this overcomes the surface tension problem, and the colour spreads out in an organic way. Add too much colour, and this can actually be reversed.. you suck dry the brush to a fine tip. This compacts the brush and it almost become 'vacuum packed', then when you dip the brush tip into the area you are working, capillary action draws away excess material.

This leads onto 'why not use water-colour type paints?'.. these aren't 'sticky' or permanent enough, also I was concerned that any future humidity in the case could cause them to run. As the surface I was applying to wasn't porous.

I needed a substance that was stable chemically, stable (longevity), and had the correct viscosity, water based when applying and easy to control. Plus, it's kinda 'organic', being tar based (I guess). I also believe that it's possibly to remove what I have done.. it would be tricky, but that was a consideration. (if it all went wrong).

Stewart
- Heuerville