Thanks for contact. I suppose if you make 100 projectiles a year, 1 litre should last you a while.
1 Litre coats about 35,000-45,000 projectiles.
I have had coating sit around for 8 years, it darkened and became more viscous, but seemed to work OK. The final colours were darker due to aging.
I have never had a situation where a user has coating left after a few months.
If your production is that low, you really should be sending your casts to a coater to coat for you. It should cost you much less and the product should be good as you have no experimental losses and need for recycling of faulty product.
Alternatively, you can get all your mates to combine their lot and do a combined batch to maximise costs and production efficiency.
I do not know what else I can tell you, but to expect a coating to last forever is a little over the top.