First time using Vallejo. I cannot get a decent spraw with it. I tried it with 20# and 25#. Used it straight out of the bottle and tried thinning a little. parts were clean. I get a real mottled finish. Any thoughts?
Thanks

If the parts are squeaky clean and the primer mixed well, it may be that it is going on too thick. Try a mist coat followed by a cover coat and see if that helps. If not, a light sanding with 800-1000 grit may get rid of the gloss finish on the plastic (that is if this is going on styrene).

Are you using their primer as in the small 17ml bottles ('normal' Vallejo) or their Polyurethane Acrylic primer? (available in 17ml to 500ml bottles?)

The Polyurethane primer needs a higher air pressure, & spray neat, ie don't dilute.

Your target needs to be clean, & if possible, key the plastic with wet&dry, wire wool, 3M pot-scrubber, etc.

Having said that, as above, I have had some success diluting The Black Polyurethane primer, using paint lacquer/(Cellulose in the UK) just to get some bite on a french kit that seems to be made of cheese instead of plastic.

What pressure?
thanks

You need a higher air pressure (from the compressor to the airbrush) for the Vallejo Polyurethane primer, it's a bit thicker than the usual varieties of paint.

AS always, practice practice... vary the distance to target, too close & it goes on wet & runs, too far & it's dry/semi-dry, creating a pebble-dash effect.

If you have a basic on/off type compressor it might not be able to produce enough air, what type do you have?

Vallejo Olive Drab primer sprayed at 18-20 psi with a Iwata HP-C Plus airbrush. The distance was about 15 cm.

what Vegard said

cleans well with alcohol