Any concerns with premixing Mission Models additives?

That's what I expected. 

To each his own but I prefer mixing in a jar or cup and then transferring to the airbrush. I played around with these paints when I got them and a little more the other day. I don't think mix ratios are that critical. What you saw on the site kinda goes along with that. As long as you don't get it watery. Maybe measure some thinner to another bottle, add some poly and give it a try. Just don't expect a long shelf life. 

Mission's response:

We do not recommend pre-mixing large batches.  Pre-mixing for long term storage could cause premature kick over or other inconsistencies. You can try it in smaller test batches but it is just as quick to mix as you go. 

I hear you. I mixed up a little and shot it 2 days ago. I used about 30% poly and around 50% thinner. It sprayed well and dried fairly quick. Just now burnished down Tamiya tape and yanked it off, no lifting. I used their black primer, gloss sea blue and gloss clear. Each color I shot 2 or 3 light coats for coverage a few minutes between. Waited 30 minutes and shot the next color.   It was just a plastic spoon so I wasn't spraying for long but it shot great, no problem. For reference,  10 drops color, 3 drops poly, 5 drops thinner. Probably more thinner in the primer, I tried using from the bottle and it's too thin for that. I switched to a pipette. 

Wingman_kz

It's been a few months since I've read them but go to their website and check the FAQ page. There may be something there about what you're asking.

Premixing isn't addressed on the website, so I sent them a message.  I'll post the reply here when I get one for all to see.

On a related note:  What the heck is up with the ratios they post?  Between their own website, instructions and literature, you can find anything from 10-30%.  I guess it probably isn't that critical.  I've been having good results with 10:3:3, but plan to play around with that a bit in both directions.

The poly additive goes bad after a while so I wouldn't pre mix and store. My poly turned into jelly after over a year or so.

It's been a few months since I've read them but go to their website and check the FAQ page. There may be something there about what you're asking. I bought a couple colors, clears, primer, poly and thinner to try but haven't done a lot with them. Haven't tried what you're asking but I've seen it mentioned somewhere. 

I have a few bottles and I mix in the ab cup too. I add 2-3 drops more of their thinner than what the instructions call for and find they spray better specially if you need the paint to be a bit translucent over a marble coat. Do use a good primer like Tamiya white or grey otherwise the paint will scratch ver easily.

I mix mine in the airbrush cup.The dropper bottles make it easy to measure out just what I need