Respirator for spray varnish on oil paints!
I have several different types , it depends on the job, the longer the job the better quality you want. I use for example painting a meat locker with an alcohol based paint I was expecting 6 hours so I paid more for a brand that had a lower resistance (work of breathing) that took all the nastys out. And similar with high VOC paints, their are times we need the really nasty paints (I say nasty because what they do to the body until cured). I am a big guy so I need a large frame mask, for a high quality mask there is really not a one size fits all. on my best mask I was professionally fit for size , they break a vile containing banana oil? I think it was because we can detect it at small amounts and found I needed a large even though I thought a medium fit well. You probably won’t need an air supplied version but I know a couple of people that work with toxic gasses that use fresh air pumps because it ends up being cheaper than regularly replacing filters, many filters expire x hours after they are opened, some allow for a total exposure time. So these things need to be considered in your choice also but start with the paint name and MFG SDS then make your decision based on that. 
I added the additional information on size because I thought I was a medium until being professionally fit , a quick test is to seal the cartridge hole and breathe in then hold your breath and see if the mask leaks air and expands it is not comfortable to do this but it was one of the quick checks before using the banana oil.
I hope this helps.