I have three guns, two of them siphon full size, one of them a detail, all of them off brand. How can I determine what size tip they are. I have sprayed some small parts with single stage paint and had good results, but I want to spray some high build primer and want to be sure I have a suitable gun. My understanding is need a 1.8 mm (or larger) tip to shoot the primer/surfacer.

check for a stamping number on the tip

there should be a stamping either on the tip or the cap. Alot of cheap guns though won't have any markings at all. you can look at the tip and see which one is bigger and use that as a primer gun. And no you dont have to have a 1.8 or bigger for primer. The smaller the tip the smoother it lays down(to a point, you get down too small and the primer wont come out well.) The bigger the tip the more material it puts down and the more peely it is. With 2k primer a 1.8-2.0 would be max that i would go, but would lean probably more towards 1.6. the smoother you lay your primer the less you have to sand and you can sand with finer grit paper to start off with which is better. you are waisting time and material sanding with course grit to get smooth. if you HAVE to do that because you are trying to fill too much(deep sandscratches, gouges etc) with the primer, you aren't preping your body work good enough. I know several people that get their body work "close" then try to get it the rest of the way with primer. It always comes back to haunt them later.
Sorry about going off on that.:lol:

Thanks for the input, I will check it out.

Most touch up guns are 99% a 1.0
As for the large ones,Try looking on the NEEDLE. Sometimes,the size is stamped on the "trigger" ferrell about 3/4 way back from the tip.
Techincal way is to remove the needle and air cap,take a new toothpick and rotate it into the front nozzle opening far enough to create a ring around the pick,Pull it out and measure it with a dial caliper,a metric one is best otherwise you have a little math to do. Yeah, I know,but it works if you "really" want to know.:lol:
Vintage is right.I "like" a 1.6-1.8 for most of my 2K primer spraying.Polyester and I drag out the hoser.
What ever the case.Allow a couple days for the primer to gas out and even after blocking,roll it out in the sun for a day "if" possiable.Primer WILL shrink and it "may" look good,but you check it in a couple of days and you'll SEE different.Trying to fill 80 gt with primer is BAD news so finish it with some 220 before the next round.