Anyone familure with a "wet spot" water filter ?

http://www.softwetspot.com/products.htm

Sure. It's the same ion-exchange resin inside that you'll find in a home softener. You back-flush, then recharge with salt flow in reverse thru the resin bed.

"Wet Spot Filter?"......completely not what I was expecting.

so... its a tube full of resin beads .. hard water goes in ..soft comes out .... The recharge Tube is empty ... the directions say to fill with water softner salt and back wash ...
hmmmm ... only 800.00 ....

Will it work when washing a black car, without wiping down after the rinse?

Yep. You can also buy a water softener at Lowe's for less that will automatically recharge and soften all the water in your house.

You can make them your self with some fittings from a hardware store ans a 1/2" pipe tap. You can buy the resin at a softener repair place or on-line. Probably make the whole thing for under $100. I've seen build plans on the net before.

I have a softener on the house and regardless of what type of salt I use we still get calcium spots, that are very hard to remove, even on the glass. Looking at a D.I. water generator(De Ionized) simular to the final rinse used before powder coating.

Mr. Clean Car-Wash System- about $35. Works just fine.

Most likely your softener isn't working as wellas it could. The resin loses effectiveness over time- someone told me it half-life is like 12 years. You may also have very hard water. DI would be unnecesary and cost prohibitive. Whole-house RO might be mre efficient.

I'd start with looking into why your softener isn't doing the job.