Problem with PollyScale flat finish.

I thought I've diluted it pretty thin, but I'll try to dilute it more, and to strain it.

 I've bought it couple of months ago. Don't know how long it was in the shop, though.

Hmmm... The only mention of year I've found on the bottle was "2000". 7 years old? Okay, looks like I've been had a bit. :) Better find a new bottle.

What you're getting looks like a combination of dusting and flocculation—a $10 word (convert to pounds on your own…) for microscopic clumping.

You need to do two things:

Dilute the paint a lot more—nearly 50 percent—to the consistency of 2 percent milk.

Strain the paint through a fine mesh screen to remove the clumps. If you have an electric stirrer, that may help, but you still need to strain it after stirring.

It works best sprayed in light coats at a higher pressure, say 15 to 20 psi. 

You didn't mention how old the paint was. Clear flats have a shorter shelf life than anything except metallics.