I use a sauce bottle that doesn’t have a separate seal in the lid. The shape of the lid clamps on the top of the bottle and doesn’t leak. The stopper cap works very well and the bottle can sit for months without much evaporation. Just give the cap a quick wipe with acetone while any spilled coating is still fresh and it cleans straight off. Don’t bother using a syringe, found stainless measuring spoons are perfect. Good shake of the bottle right before pouring stirs up any settled coating. Once coating measured and poured, simply put the spoon in a jar and cap when not being used. Once there is a bit of coating buildup on the bottom I thin with acetone and that can be used again.
If you are experimenting with many colours then using recycled containers would be a cheaper alternative.
I’ve been using this bottle for at least two years.
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